ABC News' Michael Falcone reports: 
 
 UPDATED: COLUMBUS, Ohio - Even if Rick Santorum wins Ohio on Super Tuesday, he won't be able to claim all of its delegates. In fact, he is at risk of forfeiting more than one-quarter of them. 
 
 In three of the state's 16 congressional districts, including two that are near Ohio's border with Pennsylvania, Santorum will lose any delegates he might have won because his campaign failed to meet the state's eligibility requirements months ago. 
 
 Those three districts alone take 9 delegates out of a total of 66 off the table for Santorum. 
 
 But it gets worse: Nine more Ohio delegates may also be in jeopardy. 
 
 Sources say that in six other congressional districts - the third, fourth, eighth, tenth, twelfth and sixteenth - Santorum submitted fewer names than required to be eligible for all three delegates up-for-grabs in each district. 
 
 That means even if he wins in those places, he might not be able to receive the full contingent of delegates. 
 
 In the three districts where Santorum did not submit a delegate slate at all, he will not be able to receive any delegates. In the six where he submitted only a partial slate, he is eligible to be awarded only the number of delegates he submitted, assuming he wins a particular district. 
 
 Chris Maloney, a spokesman for the Ohio Republican Party, said the leftover delegates will be considered "unbound" and the campaigns will be able to file a petition with the state party to claim them. Once such a petition is filed, Ohio GOP Chairman Kevin DeWine is required to impanel a "committee on contests" composed of three members of the Ohio GOP's central committee to sort out the delegate awards. 
 
 What's clear is that Santorum will be competing in Ohio on Tuesday handicapped by the fact that he is ineligible to receive nine delegates and perhaps as many as nine more, or more than one-quarter of the state's delegates. 
 
 Worse yet, some of these problem districts are in areas of the state where Santorum is expected to do well. The sixth congressional district, for example, hugs Ohio's eastern border with West Virginia and Pennsylvania - the state Santorum represented in Congress. The thirteenth district, which includes Akron, is nearby. 
 
 Notably, Santorum plans to spend election night in old steel town of Steubenville, Ohio, located in the sixth district, even though he has no chance of collecting any of the district's three delegates. 
 
 The bar to file what's known in election parlance as a "full slate of delegates" in each district was not particularly high. Candidates were required to submit the names of three delegates and three alternates per district. 
 
 According to the Ohio GOP, the Santorum campaign is missing the names of one delegate in the state's third district, two in the fourth, one delegate in the eighth, two delegates in the tenth, one delegate in the twelfth and two delegates in the sixteenth. Santorum's opponents - Mitt Romney, Ron Paul and Newt Gingrich - each filed full slates in all of Ohio's districts as did two candidates who are no longer in the race, Rick Perry and Jon Huntsman. 
 
 In other words, Santorum needed to submit 48 names in order to be eligible to compete for all of state's district delegates outright. 
 
 Santorum is facing a high-stakes contest with Romney in Ohio. A Quinnipiac University poll released on Friday showed the former Pennsylvania senator with a slim four percentage point lead over Romney in the state, 35 percent to 31 percent. Roughly one-third of likely Republican primary voters said they could still change their mind before Tuesday. 
 
 And as the Republican nominating contest becomes as much a battle for delegates as it is for momentum, Santorum's difficulties in Ohio offer another window into the organizational challenges his campaign has faced throughout the primary season. Santorum is not on the ballot in Virginia, which also holds a primary on Super Tuesday and where each candidate was required to submit 10,000 signatures, including 400 from each of the state's 11 congressional districts. 
 
 The eligibility requirements in Ohio were comparatively easier, but at the time Santorum's rivals were putting together their delegate slates in December, Santorum was crisscrossing Iowa in a pickup truck ahead of the caucuses, which he ended up winning by a hair. The deadline for submitting the slates in Ohio was the last week of December. 
 
 There are a total of 63 delegates up for grabs in Ohio on Tuesday, 48 of them are awarded proportionally based on who wins the popular vote in each Congressional district - three per district - and 15 will be awarded to the candidate who wins a majority of votes in the state. However, if no candidate surpasses 50 percent of the vote, the "at-large" delegates are awarded proportionally to each candidate who received more than 20 percent of the statewide vote. Three additional party leaders will act as unbound delegates at the Republican National Convention, but they will not be awarded on Tuesday. 
 
 The Romney and Santorum campaigns offered vastly different takes on the delegate situation in Ohio. 
 
 "The attempt by the establishment to to deceive the voters of Ohio and further their hand-picked candidate will be met with resistance on Tuesday. I want to be clear, Rick Santorum's name will appear on every ballot in the state of Ohio and every vote cast will go towards his At-Large Delegate Allocation," Santorum campaign communications director Hogan Gidley told ABC News. "As it relates to individual Congressional Districts, it's clear we aren't the establishment hand-picked candidate and back in December we were a small effort focused on Iowa. Now that we've won several states Obviously much has changed and we feel confident that we will do well in both the delegate and popular vote count on Tuesday." 
 
 Romney campaign spokesman Ryan Williams said that the Santorum campaign's shortcomings in the state was a sign of weakness. 
 
 "Rick Santorum has failed to get on the ballot in Virginia, has failed to file full delegate slates in Tennessee, New Hampshire and Illinois, and has failed to submit enough delegates in several Ohio congressional districts," Williams said in a statement. "The fact that he cannot execute the simple tasks that are required to win the Republican nomination proves that Rick Santorum is incapable of taking on President Obama's formidable political machine." ||||| UPDATED with new information 
 
 WASHINGTON -- Rick Santorum was already known as starting from a deficit, delegate-wise, in Ohio. He failed to qualify for any district delegates in three Ohio congressional districts because he didn't turn in delegate names there. 
 
 But his delegate troubles go deeper. According to the Ohio Republican Party tonight, the former Pennsylvania U.S. senator filed incomplete delegate slates in six additional Ohio districts. 
 
 Altogether, this means Santorum, who until this week had a fair lead in polls in the Republican nominating race, could be ineligible for 18 Ohio district delegates. 
 
 Ohio has 66 delegates total, 63 at stake next Tuesday. The candidate with the most delegates wins. Santorum therefore goes into the Ohio primary election with a 29 percent deficit. 
 
 What will happen if he wins in a district where he failed to allocate a full slate of three delegates? 
 
 In the short term, he will be eligible to take only the delegates he has already allocated in that district, the party says. Yet he will have won that district -- so the unallocated delegates will not be awarded to anyone else, either. 
 
 After all, they did not win them. 
 
 "On Super Tuesday, if Sen. Santorum were to carry a district where he has not seated a full delegate slate, he will be awarded delegates where he has submitted delegate names," said Ohio Republican Party spokesman Chris Maloney. "And the additional delegates in that district will be unallocated." 
 
 This almost certainly sets up challenges, if not from Santorum then from political rivals such as Mitt Romney or Newt Gingrich. 
 
 The state party says it has not faced this situation before, but it has been reviewing its bylaws and is prepared, if there is a challenge, to convene what it calls a "committee on contests." 
 
 State Republican Chairman Kevin DeWine would appoint three party members to the committee, and they would make a recommendation to the state party's central committee, which would rule. 
 
 If Santorum or another candidate did not like the ruling, he could appeal to the Republican National Committee. 
 
 ABC News' The Note broke the story this evening. 
 
 The Ohio Republican Party says it discovered Santorum's problem this week. While candidates were supposed to file their delegate slates with the Ohio secretary of state as well as the state party, Santorum only filed his with the secretary of state. The secretary of state, however, has no authority over delegate counts. That office merely needs to make sure that a candidate has qualified with a bare minimum of delegates to get on the ballot. 
 
 Secretary of State Jon Husted's office did that, according to the state Republican Party. In the three congressional districts where Santorum filed no slates of delegates whatsoever, Husted's office ruled that Santorum would be ineligible to be on the ballot for district-delegate purposes. This has been known and reported for some time. 
 
 Santorum's name will be on ballots in those districts for purposes of another kind of delegate, however, based on the statewide popular vote. In other words, all other candidates will have their names on the ballot twice -- once for district delegates, once for the state delegates. But there are far fewer state delegates to be won than there are district delegates. 
 
 The Ohio Republican Party, saying it never got a slate of delegates from the Santorum campaign, got a copy from Husted's office, according to one official. The party needed to review delegate slates in preparation for next week's election. It was only then that it realized there was a problem and began informing all the candidates this week. 
 
 Ohio has 16 congressional districts, or will for purposes of this election. (It now has 18 but will lose two at the end of 2012.) Romney, Gingrich and Ron Paul have slates of three delegates in each. But here is how Santorum's slates shake out: 
 
 District 1: 3 delegates 
 
 District 2: 3 delegates 
 
 District 3: 2 delegates 
 
 District 4: 1 delegate 
 
 District 5: 3 delegates 
 
 District 6: 0 delegates 
 
 District 7: 3 delegates 
 
 District 8: 2 delegates 
 
 District 9: 0 delegates 
 
 District 10: 1 delegate 
 
 District 11: 3 delegates 
 
 District 12: 2 delegates 
 
 District 13: 0 delegates 
 
 District 14: 3 delegates 
 
 District 15: 3 delegates 
 
 District 16: 1 delegate 
 
 This could get interesting, and difficult, for Santorum because his failure includes districts where he might otherwise run well. 
 
 He was already at a 3-delegate deficit, for example, in the Steubenville area (District 13) and another 3-delegate deficit further south on Ohio's eastern border (District 6), areas where steel workers and others might remember him from his days serving Pennsylvania. He has campaigned in that region anyway; Santorum's grandfather was a coal miner, as are some of the area's residents. 
 
 And he has his other 3-delegate deficit on the West Side of Cleveland and some of its suburbs (District 9). While Romney has been expected to perform better in Northeast Ohio in general, Santorum's social conservative message could appeal to Catholic voters in the western neighborhoods and suburbs. 
 
 But Santorum also has diminished his chances in socially conservative counties like Allen, Shelby and Auglaze (District 4) and -- this is noteworthy -- Montgomery, Greene and Fayette counties (District 10). The latter two can be considered Mike DeWine country. DeWine got his political start in Greene and still has his home there. As state attorney general, DeWine is Santorum's highest-profile supporter in Ohio. Akron, too (District 16), is now problematic for Santorum. 
 
 A Santorum spokesman has not returned a message seeking comment. But Santorum is in Lake County tonight, where reporters are certain to ask about this. The campaign of his chief competitor, Mitt Romney, has already pounced on Santorum's apparent error. ||||| (Credit: AP Photo/Steven Senne) 
 
 Former Massachusetts Gov. Mitt Romney cruised to victory in the Illinois Republican presidential primary on Tuesday night, defeating his closest rival, former Pennsylvania Sen. Rick Santorum, by double digits. 
 
 
 
 With nearly all precincts reporting, Romney had 47 percent of the votes cast to Santorum's 35 percent. Ron Paul was in third with 9 percent, and Newt Gingrich in fourth with 8 percent. Turnout was light in the state. 
 
 
 
 Live Illinois GOP Primary Results 
 
 In victory remarks in Schaumburg, Illinois, Tuesday night, Romney kept his focus squarely on President Obama -- not his Republican primary opponents. 
 
 
 
 "Three years of Barack Obama have brought us fewer jobs and shrinking paychecks, but many of us believed we were in danger of losing something even more than the value of our homes and our 401(k)s," he said. "After years of too many apologies and not enough jobs, historic drops in income and historic highs in gas prices, a president who doesn't hesitate to use all the means necessary to force through Obamacare on the American public, but leads from behind in the world. It's time to say these words, this word: enough. We've had enough." 
 
 CBS News exit polls found that Romney, whose base of support was in Chicago and its suburbs, performed well among college graduates, Catholics and voters who see the economy as the most important issue facing the country. He also had a small edge over Santorum among those identifying as conservative. Santorum led Romney among white evangelicals and those identifying as very conservative. 
 
 Exit polls found that 42 percent of Illinois Republican voters said Romney is not conservative enough, while a roughly equal percentage said his positions were about right. Forty-six percent said Santorum's positions were about right, while 34 percent said he was too conservative and 13 percent said he was not conservative enough. 
 
 
 
 Illinois Exit Poll 
 
 Illinois primary results by county 
 
 Romney was backed in Illinois by a massive spending advantage over the other candidates for the GOP nomination, as he has been in other states. According to a Campaign Media Analysis Group analysis, Romney and the super PAC backing him outspent Santorum and the super PAC backing him 18-1 on television ads in Illinois. Romney and his backers spent more than an estimated $3 million on television ads to Santorum's estimated $167,090. 
 
 Also boosting Romney was the perception that he has the best chance to defeat President Obama in November. Exit polls showed that 60 percent said Romney was the most electable candidate, compared to 23 percent for Santorum and nine percent for Gingrich. 
 
 In the exit polls, Romney led his rivals on the question of which candidate best understands the problems of average Americans, with 36 percent support to Santorum's 29 percent and Gingrich's 16 percent. In the Ohio primary, the wealthy former CEO was at only 22 percent on this question. 
 
 
 
 How Romney won the Illinois primary 
 
 Romney is now expected to take to take the lion's share of the 54 delegates up for grabs in Tuesday's primary, which are allocated proportionately. The former Massachusetts governor is focused on getting the 1,144 delegates he needs to clinch the nomination before the convention. His rivals are largely trying to keep Romney from that threshold, with the hope of ultimately securing the nomination at a contested convention. 
 
 
 
 Santorum was not eligible for ten delegates in Illinois because his campaign failed to identify supporters who could serve as delegates in four congressional districts before the election. Neither Gingrich nor Paul seriously contested the state. 
 
 
 
 CBS News now estimates that Romney has 534 delegates, more than his three rivals combined. Santorum has 218 delegates in CBS News estimates, while Gingrich had 120 delegates and Paul had 42 delegates. 
 
 Estimated Republican Delegate Scorecard 
 
 Santorum said before Election Day that he was holding out hope for a surprise victory driven by the more rural, conservative voters in the southern and western part of the state, saying it would "really send shockwaves" through the Republican establishment. He held four rallies and submitted to 19 interviews in Illinois on Tuesday in a last-ditch effort to spur turnout among his largely-downstate supporters before heading to his home state of Pennsylvania, where voters go to the polls on April 24. 
 
 "We won the areas that conservatives and Republicans populate, and we're very happy about that," Santorum said Tuesday in Pennsylvania, after acknowledging his loss in Illinois . "We're happy about the delegates we're going to get too." 
 
 Santorum's campaign argued earlier Tuesday that the delegate estimates by CBS News and other news outlets overstated Romney's advantage, since they are doing well in certain county, district, and state conventions that will send delegates to the Republican convention. It also plans to challenge the decision by Florida and Arizona, a pair of states won by Romney, to award delegates in a "winner-take-all" fashion. A successful challenge would reduce Romney's delegate advantage. 
 
 
 
 In addition to Pennsylvania, Santorum expects to do well in the Louisiana primary on Saturday and go on to win in Arkansas and Kentucky on May 22. He has also set his sights on Texas, which offers 155 delegates and which holds a primary on May 29. 
 
 In the short term, however, the calendar offers Romney far better opportunities to garner delegates. There are primaries in April in the District of Columbia, Maryland, Wisconsin, Connecticut, Delaware, New York, Pennsylvania and Rhode Island, most of which Romney is expected to win. In addition to Pennsylvania, Santorum's best chance for victories in April come in Maryland and Wisconsin. 
 
 2012 Republican Primary Calendar 
 
 The victory in Illinois is the third win in the Midwest for Romney, who attacked Santorum in the run-up to Tuesday's contest as having "the same economic lightweight background the president has." Santorum, who has been hammering Romney over his past support for an individual health care mandate and his past as a "Wall Street financier," shot back that "If Mitt Romney's an economic heavyweight, we're in trouble." 
 
 The Santorum campaign has argued that if Gingrich left the race it would open the door to Santorum winning the nomination on the strength of conservative support. In exit polls Tuesday, Illinois Republican voters were asked how they would vote if only Romney and Santorum were in the race. Forty-seven percent said they would support Romney, 41 percent chose Santorum, and 11 percent said they would not have voted. 
 
 In a statement Tuesday night, Gingrich took aim at Romney for spending so much to win and vowed to stay in the race, saying "we need a nominee who offers powerful solutions that hold the president accountable for his failures." 
 
 
 
 With the nomination seemingly within reach, Romney is expected to aim his rhetoric squarely at the president in preparation for the general election. His rivals, meanwhile, will faced increased pressure to leave the race so that the party can rally around a nominee and leave behind what has been a bruising primary fight. 
 
 "Tonight was a primary, but November is the general election," Romney said Tuesday evening. "And we're going to face a defining decision as a people. Our choice will not be about party or even personality. This election will be about principle. Our economic freedom will be on the ballot. 
 
 Santorum looks ahead to Pennsylvania and Louisiana 
 
 Romney on Obama: "We've had enough"
– Rick Santorum may be at the top of the polls in Ohio, but even if he wins there, he may not win. Because his campaign missed several eligibility deadlines, he could lose up to 18 of the state's 63 available delegates, report the Cleveland Plain Dealer and ABC News. In nine districts, Santorum's campaign filed either incomplete delegate slates or none at all. Even worse, several of those problematic districts are areas where Santorum was expected to perform well. Santorum is not even on the ballot in Virginia, and failed to file full slates in Tennessee, New Hampshire, and Illinois, but the eligibility requirements in Ohio were considered easier. Santorum's campaign said that because he is one of the leading candidates in the GOP primaries, he should—and will—receive the full delegate count he wins on Tuesday, but the Romney campaign disagreed. “The fact that he cannot execute the simple tasks that are required to win the Republican nomination proves that Rick Santorum is incapable of taking on President Obama’s formidable political machine," said the campaign. The state party would have the final say.