Republicans claimed Senate seats vacated by Democrats in Georgia, Florida, Louisiana, North Carolina and South Carolina. In South Dakota, Minority Leader Thomas A. Daschle fell to a challenger closely identified with President Bush. And a GOP-crafted House redistricting plan in Texas led to the defeat of four veteran Democratic lawmakers and helped Republicans expand their majority by three and possibly four seats.

These changes have the potential to reduce the importance of Republican moderates, especially in the Senate, and embolden conservatives in the White House and elsewhere, these analysts said. But they also might heap unrealistic expectations on Senate Majority Leader Bill Frist (R-Tenn.), who still lacks a filibuster-proof majority as he weighs a 2008 presidential bid.

The GOP's bare Senate majority of 51 members will grow to 55, but the impact goes beyond mere numbers.

Replacing Sen. Ernest F. Hollings (D-S.C.), often a bridge between the two parties on spending and deficit questions, is Rep. Jim DeMint, whose call for abolishing federal income, payroll and estate taxes is considered extreme even by some fellow Republicans. Succeeding Sen. John Breaux (D-La.), a key broker on sticky issues such as Medicare, is mainstream Republican Rep. David Vitter, who seems unlikely to play such a bipartisan role.

"Regrettably, we have seen an erosion in the Senate of centrists on both sides of the aisle," said Sen. Olympia J. Snowe of Maine, a Republican moderate whose leverage may drop substantially in the next Congress. She said she hoped Bush will push for cooperation between the two parties.

Another GOP moderate, Sen. Lincoln D. Chafee (R.I.), expressed even deeper disappointment, telling the Providence Journal he would not rule out switching to the Democratic Party.

While Frist celebrated the victories that will leave only four Democratic senators in the former Confederacy, some political scholars noted he must oversee a diverse delegation that still has enough moderates to occasionally frustrate Bush's agenda. But in light of Tuesday's election results, conservatives in the House and White House may show less patience with roadblocks to drilling for oil in Alaska wildlife refuges or limiting civil liabilities for doctors and others.

"The locus of power has moved dramatically to the right in the Senate," said Marshall Wittmann, a senior fellow at the Democratic Leadership Council and former staff member for the Christian Coalition and Sen. John McCain (R-Ariz.). Whereas Democratic and moderate Republican senators often could derail or temper conservative initiatives from the House, he said, "that now becomes much more difficult."

The trick for Frist, he said, will be to nudge Congress's agenda to the right without appearing to cede control to hard-core partisans such as House Majority Leader Tom DeLay (R-Tex.). With a beefed-up majority that still lacks the 60 votes needed to block a filibuster, Wittmann said, "Frist has a dilemma that he may luxuriate in or he may rue."

His best option, several analysts said, may involve appealing to Democrats who will view Tuesday's results as a warning to those who try to thwart the president's agenda.

Frist's GOP colleagues already are raising expectations. Sen. George Allen (R-Va.), head of the National Republican Senatorial Committee, told reporters that voters should expect a "reinvigorated, stronger Republican majority in the Senate" that will push aggressively for new judicial appointments and an array of measures the Democrats have stymied in recent years, such as a far-reaching energy bill.

According to Allen, Americans said, "We don't want partisan bickering, we want action."

But Frist's challenge is greater than many people realize, said James Thurber, director of American University's Center for Congressional and Presidential Studies. The election results, he said, will embolden DeLay and other House leaders to accelerate their strategy of "sending fairly conservative and edgy legislation to the Senate." Awaiting it there will be staunch conservative newcomers such as DeMint and senator-elect Tom Coburn (R-Okla.), considered the most right-leaning of the nine new senators elected Tuesday.

But the GOP caucus still includes Snowe, McCain, Chafee and other moderates such as Susan Collins of Maine and George Voinovich of Ohio. When those Republicans side with Democrats, Thurber said, "The Senate continues to be deadlocked over issues the president will want to deal with, such as Social Security and even energy."

"The appointment of a Supreme Court justice," which seems likely in Bush's second term, "will be the major battleground," he said. Voters may find it hard to understand -- and forgive -- if Democrats mount a filibuster that Frist cannot overcome.

The next Senate will have 55 Republicans, 44 Democrats and a liberal-leaning independent. Republicans achieved the four-seat gain by winning all but one of Tuesday's nine closely watched races. They grabbed the five open southern seats, toppled Daschle and hung on to embattled seats in Oklahoma, Alaska -- where Sen. Lisa Murkowski (R) fended off former Democratic governor Tony Knowles -- and Kentucky, where Sen. Jim Bunning (R) survived a strong challenge from Democrat Daniel Mongiardo. As expected, they lost their open seat in Illinois. The only Republican disappointment came in Colorado, where Democrat Ken Salazar defeated beer magnate Pete Coors to replace retiring Sen. Ben Nighthorse Campbell (R).

The GOP success was especially galling to Democrats because they felt they offered strong nominees in every race except Georgia's. But just as Thune did to Daschle in South Dakota, the southern Republicans linked their opponents to John F. Kerry, Hillary Rodham Clinton and other symbols of Democratic liberalism at every opportunity. Democratic nominees such as Betty Castor in Florida, Erskine B. Bowles in North Carolina and Inez Tenenbaum in South Carolina portrayed themselves as independent leaders concerned mainly about their home states, but voters apparently were unconvinced. Castor was narrowly defeated by Republican Mel R. Martinez, a former housing and urban development secretary in the Bush administration.

Democrats were equally despondent on the House side, though they insisted that voters will hold Republicans accountable if they overreach. Minority Leader Nancy Pelosi (D-Calif.) told reporters: "Any doubt in anyone's mind about who's in control has now been removed. . . . The responsibility to get a job done rests on them."

The House elections left the GOP with at least 231 seats, the Democrats with 201 and one independent, with two Louisiana contests awaiting a Dec. 4 runoff. In Texas, where DeLay engineered a bitterly partisan redistricting, Democrats lost two of their most senior members, Martin Frost and Charles W. Stenholm.

In Connecticut, Democrats failed to unseat two GOP moderates, Reps. Christopher Shays and Rob Simmons. The election underscored the GOP's staying power, marking the largest number of House Republicans elected since 1946.

Senate Republicans are still five votes shy of the 60 needed to cut off Democratic filibusters, but some said Bush's victory and GOP gains in Congress may give Democrats second thoughts about blocking as many Republican initiatives -- including judicial nominations as well as legislation -- as they blocked during the last two years.

"With 55 Republicans in the Senate and especially with the defeat of Senator Daschle, judicial nominations will be an area where [Democrats] will have to reassess their obstructionism," said Sen. John Cornyn (R-Tex.).

But Cornyn and others also suggested that a lot hinges on Bush. "The president has a great opportunity here to strike a positive tone, and that could do a lot to cool tempers and partisan passions in the Senate," Cornyn said.