(CNN)  -- Republicans' tough task of taking back control of the Senate next year now looks even tougher.





Sen. Pete Domenici, R-New Mexico, says he will retire at the end of his term.





Republican Sen. Pete Domenici of New Mexico announced Thursday that he will not seek re-election.





"For the past 35 years my friends and neighbors have given me the honor of allowing me to serve them as their United States senator," he said. "Who could ask for anything more?





Domenici joins three other Republican senators who also are retiring at the end of their terms next year.





Democrats took back both houses of Congress last November, but hold thin majorities. In the Senate, they have a 51-49 margin. Their advantage in the House of Representatives is 233-202.





Of 34 Senate seats being contested next year, Republicans hold 22. With the four senators retiring and four others facing difficult re-election bids, the numbers are stacked against GOP hopes of recapturing the upper chamber of Congress.   Read more about the senators who are retiring »





Domenici, a six-term senator, earlier had hinted he would run for re-election despite some criticism of his role in the controversial firings of U.S. attorneys.





One of the prosecutors, David Iglesias, contends Domenici and U.S. Rep. Heather Wilson, R-New Mexico, wanted him to push harder on a corruption investigation of state Democrats before the 2006 midterm elections.





Iglesias said he felt "leaned on" when Domenici called him to inquire about when indictments would be brought. The senator, who nominated Iglesias for the post, apologized for making the call but said he never pressured Iglesias.





Domenici said he had complained to Justice Department officials about the pace of prosecutions in Iglesias' office. Then-Attorney General Alberto Gonzales said Domenici had complained to him about Iglesias in the fall of 2005, saying the U.S. attorney "was in over his head."





Iglesias, a Naval Reserve lawyer listed in a Justice Department evaluation as a "diverse up-and-comer" before 2006, was told to resign that December.





Domenici, 75, was first elected to the Senate in 1972 and hasn't faced a tough re-election battle since then. But New Mexico's other senator, Jeff Bingaman, is a Democrat, as is Gov. Bill Richardson, who is running for president, and Democrats are likely to pour a lot of resources into trying to capture Domenici's seat.





Republicans also face tough competition to hold onto the seats of retiring senators in Nebraska, Virginia and Colorado.





In Nebraska, former Agriculture Secretary Mike Johanns has told friends he will run for retiring Sen. Chuck Hagel's seat. Johanns is a former Nebraska governor, and Hagel said he has encouraged him to run.





Nebraska Attorney General John Bruning had planned to run against Hagel and still could run against Johanns for the Republican nomination. Two other Republicans are also in the hunt for the open seat.





Former Democratic Sen. Bob Kerrey also is considering running for Hagel's seat. President Bush handily carried Nebraska in his 2004 re-election, but an unpopular war and an unpopular president could give the Democrats hope next year. The state's other senator, Ben Nelson, is a Democrat.





In Virginia, where longtime Republican Sen. John Warner is retiring, Democratic former Gov. Mark Warner, no relation to the senator, intends to run for the open seat. Mark Warner's term as governor ended in January 2006, and he left office with high favorable ratings.





Democrats have won three major statewide elections there this decade. In 2006, Democrat Jim Webb ousted Republican Sen. George Allen by a margin of fewer than 10,000 votes, or less than half a percentage point. Webb's victory helped give Democrats their slim Senate majority.





On the Republican side, there could be a bitter primary fight between Rep. Tom Davis, a moderate from northern Virginia, and conservative former Gov. Jim Gilmore, who dropped out of the presidential race earlier this year.





In Colorado, Sen. Wayne Allard announced this year that he
How many seats do Republicans hold? How many Republican senators have announced retirement? Who holds the majority? What is the senator announcing? Who holds majority?
22. four Democrats he will not seek re-election. Democrats
question: How many seats do Republicans hold?, answer: 22. | question: How many Republican senators have announced retirement?, answer: four | question: Who holds the majority?, answer: Democrats | question: What is the senator announcing?, answer: he will not seek re-election. | question: Who holds majority?, answer: Democrats