The unexpected retirement of Rep. Mike Ferguson (R-N.J.) marks the latest in an exodus of moderate Republicans that is pushing the caucus in a rightward direction and could potentially cripple the party's chances of winning back seats in swing districts next year.
Of the 17 Republican House members to announce their retirements this year - Ferguson joined the club on Monday - eight have built reputations on Capitol Hill as centrists willing to work with Democrats to get legislation passed. Political observers warn that those are exactly the type of candidates the GOP needs to regain its congressional majority.
"It should be an area of deep concern to Republicans of all stripes. Once you lose the vital center, then you begin to lose the claim that you are the majority party," said former Rep. Sherwood Boehlert, a moderate Republican who retired in 2006.
He said that in more than four decades in political life, he's never seen "a higher degree of partisanship or a higher level of intolerance for another point of view."
Most of the moderates who have decided to leave Congress have won their districts comfortably, even in last year's Democratic wave. Their reasons for leaving range from wanting to spend more time with family - Ferguson's explanation - to a simple desire to do something else.
But each also faced a growing ascendancy of conservatives in both the House and Senate GOP caucus as well as a national environment that would seem to favor Democrats.
"It's not a good time to be a moderate in American politics," said Rep. Tom Davis (R-Va.). "Ask Joe Lieberman."
"The money has moved away from the parties, who used to be the enforcement mechanisms, to groups on the extreme right and left, and it's killing us," Davis said.
Davis said moderates were increasingly frustrated about their ability to make a difference in the current partisan age. "And they have to fight even harder to hold onto these seats," he said.
In 2006, moderate House Republicans took the brunt of the Democratic congressional landslide. Former GOP Reps. Nancy Johnson (Conn.), Rob Simmons (Conn.), Gil Gutknecht (Minn.), Joe Schwarz (Mich.) and Charlie Bass (N.H.) all were ousted by Democratic challengers.
Ferguson, who represents a suburban district in the Garden State, has been known for siding with Democrats on issues like raising the minimum wage. He has a strikingly similar profile to some of the other Republican retirees.
Retiring Rep. Jim Ramstad (R-Minn.) worked with liberal Democrat Patrick J. Kennedy (D-R.I.) to promote legislation to ensure greater coverage for mental illness. Rep. Deborah Pryce (R-Ohio), despite being part of leadership, took centrist positions on social issues, such as abortion rights and stem cell research.
And Rep. Ray LaHood (R-Ill.), one of the first Republicans to announce his retirement, is well-known for his collegial attitude toward his colleagues across the aisle.
With all the seats now open, Democrats are making a concerted push to win over districts that they view as newly winnable - as well as trumpeting to moderate voters that these retirements show that the GOP is lurching rightward.
"Moderates know if the Congress is going to be in Republican hands, it's going to be through broadening the party and winning back the districts we lost in the last election," said Bass, the former GOP congressman from New Hampshire.
On the Senate side, the majority of the retirements come from longtime members who maintained independent credentials. Three of the five retiring Republican senators- John Warner (Va.), Chuck Hagel (Neb.) and Pete V. Domenici (N.M.) - have publicly split with the Bush administration on key issues, most notably the Iraq war.
Both Warner and Domenici are leaving behind seats that the Democrats are aggressively contesting and have solid chances of winning.
Meanwhile, conservative clout is rising in most recent Republican battles - both on the Hill and on the campaign trail.
In the House, the conservative Republican Study Committee has grown nearly 50 percent in the past five years, and now has more than 100 members.
Twelve of the 15 freshman Republican House members joined the organization this year, a clear sign that the small rookie class of 2006 Republicans still believes in a conservative future. The RSC has taken the lead on GOP appropriations strategy, successfully upholding President Bush's veto of the children's health insurance bill and the Labor, Health and Human Services spending measure.
The RSC's Senate counterpart, known as the Republican Steering Committee, is seeing its influence on the rise now, meeting regularly to plan spending strategy with GOP leaders. They also have turned a once quixotic crusade against earmarks into a party-wide effort.
In contrast, the moderate Republican Main Street Partnership has seen its membership decline 20 percent, from 59 lawmakers in the last Congress to 47 this year.
And seven of those moderates are retiring, further diminishing the power of the middle.
Meanwhile, on the campaign trail, the anti-tax, fiscally conservative Club for Growth's political action committee has successfully spent millions to help elect ideologically uncompromising Republicans in competitive primaries.
Club for Growth Executive Director David Keating dismissed the suggestion that his group has pressured any of the retirees to step down. "We haven't made any noise yet," he said.
"I don't think there is a question that economic conservatives are on the upswing," Keating said. "The American people don't want any part of raising taxes."
But Boehlert said that with the exodus of longtime moderates, polarization is on the rise and less legislation is getting passed.
"You've got the Club for Growth, you've got MoveOn.org. What does that produce? Not much," Boehlert said. "You've got a lot of heat being generated but not much light. And the polarization's still there."
Patrick O'Connor contributed to this report.