Marco Rubio appeared on a Sunday talk show this month to say something remarkable. The Republican running for Florida's Senate seat suggested we reform Social Security by raising the retirement age for younger workers. Florida is home to 2.4 million senior citizens who like to vote. The blogs declared Mr. Rubio politically suicidal.

The response from Mr. Rubio's primary competitor, Gov. Charlie Crist, was not remarkable. His campaign slammed Mr. Rubio's idea as "cruel, unusual and unfair to seniors living on a fixed income." Mr. Crist's plan for $17.5 trillion in unfunded Social Security liabilities? Easy! He'll root out "fraud" and "waste."

Let's talk Republican "civil war." Not the one the media is hawking, that pits supposed tea party fanatics like Mr. Rubio against supposed "moderates" like Mr. Crist. The Republican Party is split. But the real divide is between reformers like Mr. Rubio and Wisconsin Rep. Paul Ryan, who are running on principles and tough issues, and a GOP old guard that still finds it politically expedient to duck or demagogue issues. As Republicans look for a way out of the wilderness, this is the rift that matters.

And it's the divide playing out in Florida, even if that's not the press's preferred narrative. In conventional-wisdom world, Mr. Rubio is the darling of an angry grass roots, surging at the expense of the postpartisan Mr. Crist.

Reformist Marco Rubio (left) vs. establishment Charlie Crist Associated Press

And woe betide the GOP, goes the storyline. It is courting disaster, repeating its mistake in New York 23, nominating radicals who can't win elections. Never mind the grass roots never did drum Mark Kirk (running for Illinois's Senate seat) out of the party. Or that Florida doesn't even fit this mold. Mr. Rubio, a Jeb Bush protégé, is hardly too conservative for his state. A recent Rasmussen poll has him beating Mr. Crist and Democrat Kendrick Meek statewide. Mr. Crist doesn't solve his Rubio problem by bolting the party.

What has attracted independents and even Democrats to Mr. Rubio is his reformist agenda, which taps into this week's Pew poll finding a historically low 22% of Americans trust government. It hasn't hurt that Mr. Crist has provided a sharp contrast with a campaign that channels the mindset that lost the GOP its majority.

On Social Security, Mr. Rubio is a supporter of Mr. Ryan's roadmap, which tackles entitlement and budget reform. Mr. Crist took the typical Washington path of refusing to acknowledge reality and then accusing his opponent of robbing granny. This is reminiscent of the GOP reluctance to embrace hard issues like health-care reform when it controlled Washington. One result is ObamaCare.

Speaking of that law, Mr. Rubio condemned the takeover. Mr. Crist dithered. While Mr. Rubio slammed the stimulus, the governor grabbed at its state bailout provisions since that was easier than cutting spending. One of these sounds like the GOP of old; one does not.

Floridians may remember 2007, when Mr. Rubio, as speaker of the Florida House, championed comprehensive tax overhaul. It was a bold idea to swap all property taxes for a flat consumption tax. The reform lowered overall taxes; even Americans for Tax Reform applauded it. Mr. Rubio's reward was to recently have Mr. Crist slam him for proposing a "massive tax increase." Now you know why Washington never embraces anything more than a "tax commission."

Mr. Crist is best-known for launching a vicious campaign against property insurers in the wake of Hurricane Katrina. Mr. Rubio pushed back, though it was unpopular. Now that the governor has succeeded in driving that industry out of his state, things look different. This divide is similar to today's GOP split over Wall Street, between those tempted to win points by punishing banks with overweening regulation and those in the Ryan camp who have no love for big business but defend free markets.

If an angry public has done anything, it's been to embolden more of these reformers to run. Pennsylvania Senate candidate Pat Toomey was a leader on Social Security reform in Congress. John Kasich, running for Ohio governor, promises to overhaul the state's decrepit tax and regulatory systems. In House races you see more candidates running on bold solutions. Yet for every budding Rubio there remains an establishment GOP member who fights earmark bans, blanches at Medicare reform, and just wants to get through the next election.

This divide is putting enormous pressure on the GOP leadership. It tastes victory this fall and is terrified of blowing it. It watched President Obama sandbag Mr. Ryan earlier this year, holding up his roadmap as an example of the terrors the GOP would impose on the nation.

At some point, GOP leaders are going to have to decide what the "new" GOP is. Principled opposition to bad Democratic policy is a legitimate strategy for the midterms. Then what? Republicans will win seats this fall. How long they remain in them will come down to which side—the establishment GOP or the reformist GOP—wins what is the real Republican civil war.

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