WASHINGTON — Republicans retained control of both the House and Senate in Tuesday's elections, extending their decade-long domination of Capitol Hill as they swept up victories across the South and unseated Senate Minority Leader Tom Daschle of South Dakota.

Republicans increased their control of the Senate, securing 55 seats in the 100-member Senate by picking up seats formerly held by Democrats in the Carolinas, Georgia, Florida and Louisiana.

Democratic Party officials had conceded all along that they faced an uphill climb in their quest for a Senate majority because the most competitive contests were being fought in states considered to be Republican strongholds.

"We've had a tough climb," Sen. Jon Corzine of New Jersey, chairman of the Democratic Senatorial Campaign Committee, said on CBS. "I'm disappointed we're probably not going to take it back."

Although the attention focused on a few closely fought Senate races, most incumbents up for reelection — including California's Democratic Sen. Barbara Boxer — coasted to victory.

In the House, Republicans hoped to expand their 22-seat advantage, building on the defeat of four incumbents in Texas as a result of redrawn congressional districts that favored Republican candidates. A key question was whether the gains in Texas would offset losses in other parts of the country.

The outcome of the battle for Capitol Hill is crucial for President Bush. The Republican victory in Congress makes it easier for him to advance his second-term agenda of more tax cuts and a Social Security overhaul.

In late results, Republican Lisa Murkowski defeated Democrat Tony Knowles in a tight race in Alaska, and Rep. John Thune, the GOP candidate in South Dakota, defeated Daschle. Republican Rep. David Vitter appeared to win more than 50% of the vote in Louisiana, avoiding a December runoff. He will replace Democrat John Breaux.

The 2004 elections seemed likely to produce a Senate slightly more diverse than the old one: It will include Democratic state Sen. Barack Obama, who was elected by a wide margin in Illinois, making him the only African American in the Senate and the first since the 1998 defeat of Carol Moseley Braun, who was also an Illinois Democrat.

Democratic state Atty. Gen. Ken Salazar won the open Senate seat in Colorado, and Republican Mel Martinez won in Florida, becoming the first Latinos in the Senate since Sen. Joseph M. Montoya (D-N.M.) retired in 1977.

The new Senate may be more conservative — and not just because there will be several Republicans replacing Democrats. Two of the newly elected GOP senators — Tom Coburn of Oklahoma and Jim DeMint of South Carolina — are vocal representatives of the GOP's more conservative wing.

The balloting for Congress was the culmination of a long, exceptionally nasty and expensive campaign. It featured relentless mudslinging in many places, including Democratic commercials that questioned the mental health of Republican Sen. Jim Bunning of Kentucky and Republican TV ads accusing Sen. Patty Murray of Washington of being soft on terrorist leader Osama bin Laden.

The candidates and political parties spent more than $445 million on the contest for the Senate — up from about $326 million in 2002, according to an analysis of Federal Election Commission data by the nonpartisan Center for Responsive Politics.

Heading into the election, Republicans controlled the Senate by a scant 51-vote majority; there were 48 Democrats and one independent, who usually voted with the Democrats. That meant Democrats needed a net gain of two seats to win a majority — or one if Kerry were elected.

But Democrats sought that small gain on a playing field that was tilted strongly against them. Of the 34 seats on the ballot this year, 19 were held by Democrats and 15 were held by Republicans. The nine most competitive races were in states that favored Bush in 2000 — including many in the South, where five Democratic retirements gave the GOP a golden opportunity to take away seats.

"We were playing on tough turf," said Cara Morris, a spokeswoman for the Democratic Senatorial Campaign Committee.

Republicans were relieved by returns from the Oklahoma Senate race, even though the state should have been an easy win for Coburn. He trailed his Democratic opponent, Rep. Brad Carson, for much of the campaign because of missteps and Democratic attacks on him as a political extremist. But he pulled ahead with an attack strategy that portrayed Carson as too liberal.

The key GOP gains came from picking up three Southern seats previously held by Democrats who retired — including DeMint's victory in South Carolina, where he succeeds Sen. Ernest F. Hollings. Democrats lost the North Carolina seat vacated by Sen. John Edwards when he became Kerry's running mate: GOP Rep. Richard Burr trailed for most of the campaign but ended up beating former Clinton administration aide Erskine Bowles by highlighting Bowles' ties to the former president. And in Georgia, Republican Rep. Johnny Isakson easily won the seat vacated by retiring Democratic Sen. Zell Miller.

The marquee battle was the powerful challenge to Daschle, who vainly struggled to turn back a challenge from Thune in a state that was strongly pro-Bush. Republicans portrayed him as the linchpin of congressional opposition to Bush's policies; Daschle emphasized how his clout had benefited his home state.

In the House, Democrats had an even steeper hill to climb. Republicans went into the election with 227 House seats; Democrats had 205, plus the support of one independent that usually voted with them; two seats were vacant.

That meant Democrats needed a net gain of 12 seats to get a majority. The road was uphill because there were fewer than two dozen truly competitive seats. The vast majority of House members were easily reelected.

But a handful of incumbents were shown the exit. Among the four Texas Democrats defeated on the new political map championed by House Majority Leader Tom DeLay (R-Texas) were two very powerful senior members: Rep. Martin Frost, a longtime member of his party's leadership and the senior member of the delegation, and Rep. Charles W. Stenholm, a leading conservative Democrat who, like Frost, was pitted against a Republican incumbent.

Democrats scored an upset of their own against a senior member: GOP Rep. Philip M. Crane, the longest-serving Republican in the House, was defeated by businesswoman Melissa Bean.