It would be hard to find two politicians more different than John B. Breaux and David Vitter.

Breaux, who is retiring from the Senate, is a centrist Louisiana Democrat. A pragmatic dealmaker, he shuttles between parties in search of legislative compromises.

Vitter, who was elected to succeed Breaux, is one of the most conservative Republicans in the House. He likes term limits, loathes gambling and rarely votes against his party or the president.

That changing of the guard is part of a broader trend emerging from the election that helps explain why the Senate -- like the greater political landscape -- has become so polarized. Many centrists are leaving Congress; unvarnished conservatives are arriving in their place.

The retirement of Breaux and several other Southern Democrats depletes even further the dwindling ranks of lawmakers inclined to work across party lines. They are being replaced largely by a younger generation of Republicans, schooled in a more uncompromising form of conservatism.

Six of the seven Republican senators-elect are former members of the House -- a far more partisan, combative institution since firebrand Republican Newt Gingrich ran the place in the 1990s. Two of the newcomers were backed by the Club for Growth, an activist group dedicated to clipping the GOP's liberal wing. Most of the newly elected senators are significantly more conservative than those they are replacing.

"It's a sea change in terms of losing the political center," said Sen. Olympia J. Snowe (R-Maine), co-chair with Breaux of the Senate's bipartisan Centrist Coalition.

The rightward shift will put pressure on the GOP moderates who remain. When Sen. Arlen Specter (R-Pa.), who is in line to head the Judiciary Committee, recently predicted that judicial nominees who opposed abortion rights would have a difficult time getting Senate approval, conservatives clamored for him to be denied the chairmanship.

The Senate makeup also presents a challenge for President Bush: Although the 2004 elections have given him more Republican lawmakers dedicated to his agenda, most of the new senators are not the kind of bipartisan coalition-builders the president is likely to need to enact his plans to overhaul the tax code and Social Security. Because those aims are more ambitious and riskier than his first-term agenda of tax cuts and expanded Medicare benefits, many analysts have said it would be even more important for Bush to seek bipartisan support.

"It will necessitate consensus building, especially on the large issues the president is talking about," Snowe said. "I don't see how it can be a unilateral approach."

With the retirement of Breaux, Bush is losing one of the very few Democrats who supported his idea of allowing workers to invest Social Security taxes in private accounts. The Louisiana lawmaker also brokered a compromise on Bush's 2001 tax cut and was one of two Democrats who had a hand in writing last year's Medicare bill.

The influx of new conservatives also could harden Senate Majority Leader Bill Frist (R-Tenn.) against compromise.

"The pressure on Frist -- to be hard-nosed and try to run over the Democrats rather than accommodate them -- will be pretty high," said Barbara Sinclair, a UCLA political scientist.

The Senate in recent years has tempered the conservatism of the House on taxes, spending and social issues, but House Republicans should find more allies in their campaigns to slash government spending, revamp the tax code and curb abortion.

But even with their wider margins in the House and Senate, Bush and GOP leaders will have to navigate internal divisions among conservatives with different priorities. Although religious conservatives may want to emphasize moral issues such as more curbs on abortion or a ban on same-sex marriage, other Republicans care more about pushing Bush's plans to overhaul the tax code and Social Security. Fiscal conservatives, meanwhile, may prove a drag on efforts to overhaul Social Security because of concerns about the high cost of making the transition to a new system.

The political center of the Senate dwindled after the 2004 elections in part because so many Southern Democrats retired. In addition to bridge-builder Breaux, Sen. Zell Miller of Georgia -- a Democrat so conservative that he spoke at the Republican National Convention in August -- is leaving the chamber. Other Southern Democrats on the way out include Ernest F. Hollings of South Carolina and Bob Graham of Florida, who are more moderate than many in their party.

"These were the guys that were the dealmakers, the middlemen, the conduit for compromise," said Ross K. Baker, a political scientist at Rutgers University.

The vanishing political center reflects a changing political map: The South, over the last generation, turned from solidly Democratic into a Republican stronghold, and the parties themselves have become more ideologically homogeneous.

There are now fewer conservative Democrats and fewer liberal Republicans to form a bridge between the parties. In 1980, there were 69 Southern Democrats -- mostly conservative -- in the House and 12 in the Senate; in the new Congress there will be only 48 in the House and four in the Senate. The caucus of liberal Republicans has been depleted over the last decade by the retirement of such influential senators as William S. Cohen of Maine, Mark O. Hatfield of Oregon and David F. Durenberger of Minnesota.

And the new generation of GOP conservatives is bringing a more brash, partisan style to the courtly Senate. A vivid example came with the 1994 election of Rick Santorum of Pennsylvania, who ousted Democrat Harris Wofford. One of Santorum's first acts as a senator was to challenge the institution's hallowed seniority system by calling for the removal of Hatfield as Appropriations Committee chairman because he had voted against a balanced-budget measure.

One exception in the class of GOP Senate newcomers is Rep. Johnny Isakson (R-Georgia), who easily won election to succeed Miller. Endorsed by the Republican Main Street Partnership, a group that backs GOP moderates, he wants to expand stem cell research and has never supported a constitutional amendment to ban abortion. However, he is a solid conservative in other respects, earning a lifetime rating of 84% from the American Conservative Union.

But other freshman senators, significantly to the right of their predecessors, are exemplars of the more confrontational conservatism by which Gingrich transformed the GOP.

After former Rep. John Thune of South Dakota triumphed over Senate Minority Leader Tom Daschle, he was one of the first to raise questions about Specter's comments on judicial nominations. He predicted that many of the newly elected Republicans would share his concerns, because they had made appointment of conservative judges a major campaign issue.

Former Rep. Tom Coburn is replacing retiring GOP Sen. Don Nickles in Oklahoma; both are strong conservatives, but Coburn's conservatism has a maverick quality. He took a leading role in trying to cut government spending in the House -- an aim that will be harder to achieve in the Senate. Coburn was endorsed by the Club for Growth in a contested primary in which he beat a more moderate Republican.

North Carolina is taking a conservative turn with the election of Rep. Richard M. Burr to replace Sen. John Edwards, who sought the Democratic presidential nomination rather than run for reelection -- and wound up as Sen. John F. Kerry's running mate. Burr was elected to the House in 1994 as part of the conservative GOP takeover of Congress. His strong antiabortion record helped him win support from religious activists.

In Florida, Bush's former housing secretary, Mel Martinez, is replacing Graham, a moderate Democrat who retired after running unsuccessfully for president. In a contested Republican primary, Martinez tacked sharply to the right and attacked one opponent for being "the new darling of the homosexual extremists" because he had supported protections against hate crimes.

Another big ideological swing is coming in South Carolina when Hollings, a Democrat who sometimes crossed party lines on deficit reduction, is succeeded by Rep. James DeMint, another candidate backed by the Club for Growth in a contested Republican primary. He has been a leading advocate of abolishing the income tax in favor of a national sales tax.

Louisiana's Vitter is a stalwart party-line voter. According to Congressional Quarterly, he voted with his party 99% of the time in 2002 and 2003. Breaux, by contrast, was known for his willingness to cross party lines and cut deals with Republicans. As a House member in 1981, he supported President Reagan's social spending cuts in return for concessions on his local interests. He explained that although his vote could not be bought, "it can be rented."

The Panetta Institute, a public policy center founded by Leon E. Panetta, President Clinton's former chief of staff who was a longtime congressman from Carmel Valley, Calif., chose to honor Breaux and Snowe for their "high standards of bipartisan leadership."