Republicans expanded their slender majority in the Senate in Tuesday’s election, toppling the Democratic leader, grabbing seats from the Democratic Party in at least four southern states and staving off challenges in other tight races.

Senate Minority Leader Thomas A. Daschle (D-S.D.) lost his seat to Republican John Thune, a former House member. With all the votes counted, the Democratic leader for nearly 10 years trailed his opponent by 4530 votes.

Additional Republican pickups came in North and South Carolina, Georgia and Louisiana, where Democratic retirements had given the GOP opportunities in its base region.

Early this morning, it also appeared that Republican Mel R. Martinez, the former secretary of housing and urban development, was headed for victory in Florida, in the battle for the seat vacated by outgoing Democratic Sen. Bob Graham.

In Alaska, another state in which Democrats held hopes of taking back a seat from the GOP, Sen. Lisa Murkowski (R) held a six-point lead over former governor Tony Knowles (D) with more than 80 percent of the precincts in.

Knowles charged that Murkowski got her Senate seat through nepotism rather than merit. Her father, Frank Murkowski, appointed her to the seat he had occupied after he was elected governor two years ago.

But Murkowski campaigned hard and argued that Alaska would benefit from keeping intact the state’s GOP team consisting of her father, herself and Sen. Ted Stevens, chairman of the Appropriations Committee.

The overall results were a resounding disappointment to Democrats, who were hoping to exploit the closeness of as many as 10 of the 34 Senate races this year to retake control of the Senate. The Republicans, who hold a slim 51-vote majority, were poised to expand their majority by at least three seats and as many as four.

Although the expanded GOP margin would still be well short of the 60 needed to push through legislation without risk of Democratic filibusters, the strengthened position could ease the passage of major legislation and the approval of judicial appointments, should President Bush secure reelection. Senate Majority Leader Bill Frist (R-Tenn.) would be far less dependent on the votes of a group of moderate Republicans from the Northeast to pass legislation.

There were two bright spots for the Democrats: the overwhelming victory of Illinois state Sen. Barack Obama in his bid to succeed retiring U.S. Sen. Peter Fitzgerald (R) and Democrat Ken Salazar’s victory over Republican Pete Coors for the Colorado Senate seat held by retiring GOP Sen. Ben Nighthorse Campbell.

A rising Democratic star, Obama easily won in Illinois, defeating Republican Alan Keyes in the first Senate race between two African-Americans.

Although Coors’s role as chairman of Coors Brewing Co. gave him instant name recognition throughout Colorado, Salazar was able to offset that advantage by capitalizing on his ties to the Hispanic community and his experience as state attorney general.

For a while yesterday, it appeared that Democrats might pick up another Republican-held seat, that of veteran Sen. Arlen Specter (R-Pa.). However, Specter, a GOP moderate who earlier this year survived a tough primary challenge from a more conservative opponent, defeated Rep. Joseph M. Hoeffel III (D-Pa.).

A surprisingly strong Democratic challenge also fell short in Kentucky, where Republican Jim Bunning defeated Democrat Daniel Mongiardo, a little known Democratic state senator.

In Oklahoma, former representative Tom Coburn (R) defeated Rep. Brad Carson (D) for the seat of retiring Sen. Don Nickles (R), spoiling Democratic hopes for a takeover there.

In the Georgia Senate race, three-term Rep. Johnny Isakson (R) defeated Rep. Denise L. Majette (D) to claim the seat of retiring Democratic Sen. Zell Miller. The maverick Miller has long been estranged from the Democrats, and he delivered the keynote address at the Republican National Convention this summer.

One Democratic Senate aide argued last night that the GOP victory should not be counted as a loss for the party, since Miller had rarely voted with the Democrats in recent years.

In North Carolina, the contest to fill the seat vacated by Democratic vice presidential nominee John Edwards had been close for weeks, but late last night Rep. Richard Burr (R) defeated Democrat Erskine B. Bowles, a businessman and former chief of staff to President Bill Clinton.

While Bowles painted Burr as being too close to special interests such as tobacco and pharmaceutical companies, Burr’s ads linked Bowles to Clinton, who is unpopular in conservative rural bastions.

In South Carolina, in the contest to replace outgoing Democratic Sen. Ernest F. Hollings, Rep. Jim DeMint (R) defeated former state superintendent of education Inez Tenenbaum (D). As a woman Democrat running for office in one of the nation’s most solidly Republican states, Tenenbaum had not been expected to do well. But she gained ground after blasting DeMint for advocating a federal sales tax.

Thirty-four of the Senate’s 100 seats were at stake in yesterday’s voting. Democrats were defending 19 seats, four more than the Republicans, including five open seats in southern states, where Republicans are on the rise.

Democrats knew they had to hold GOP gains in the South to a minimum if they were to retake control of the Senate, now split among 51 Republicans, 48 Democrats and one Independent who sides with the Democrats.

But that proved too difficult to do. Among the casualties were three women Democrats, all seeking to make history by representing southern states in the Senate. In the end, Tenenbaum, Majette and Betty Castor, a Democratic moderate from Florida, all apparently fell short.

No campaign was more bruising than the one in South Dakota between Daschle and Thune. The battle featured an unprecedented barrage of attack advertisements and spending that broke records for Senate races.

With the exception of Bunning and Daschle, most sitting senators faced little serious opposition, regardless of their political leanings. The power of incumbency was aiding conservative Republicans such as Sen. Judd Gregg (N.H.), Richard C. Shelby (Ala.) and Sam Brownback (Kan.) and liberal Democrats Patrick J. Leahy (Vt.), Ron Wyden (Ore.) and Byron L. Dorgan (D-N.D.).

No race produced odder twists and turns than the one between the 73-year-old Bunning, a former major league pitcher elected to the Baseball Hall of Fame, and Mongiardo, 44, who had been little known until the last few weeks.

In one of a series of gaffes, Bunning suggested that his opponent looked like one of Saddam Hussein’s sons. On other occasions he revealed that he had not read a newspaper in six weeks and said the United States was attacked by terrorists on Nov. 11.

In Florida, Martinez had been handpicked by Bush’s top political adviser, Karl Rove, used blistering advertisements and hitched himself closely to the president and his brother, Florida Gov. Jeb Bush.

Candidates raised record amounts of money, and the airwaves were also filled with ads financed by party committees and outside advocacy groups, including antiabortion and abortion rights groups, labor unions, environmentalists and the National Rifle Association. In the hotly contested Senate race in sparsely populated South Dakota, spending by the candidates totaled more than $50 per voter.

The Senate has been closely divided since 2000, mirroring the country’s narrow political divide during the period. The 2000 election ended with a 50-50 split, with Vice President Cheney breaking the tie.

But Sen. James M. Jeffords (Vt.) left the GOP, became an independent and lined up with the Democrats, giving them a one-vote edge.

The Senate remained in Democratic hands until the 2002 elections, when Republicans won their current 51-vote majority. But, with GOP moderates joining Democrats on some critical issues and Republicans usually unable to muster the 60 votes needed to break Democratic filibusters, the Senate remained a sharply divided institution.

While the Democrats’ goal was to take back control of the Senate, Republicans pinned their hopes on a bigger majority, enough to withstand defections and strengthen their hand in trying to fend off filibusters that Democrats used to block some Bush legislative initiates and judicial nominations.

To a large extent, the campaigns focused on traditional issues such as education, health care and taxes and on local matters, such as tobacco in North Carolina, oil drilling in the Arctic National Wildlife Refuge in Alaska and prairie dogs in South Dakota. But the war in Iraq and concern over terrorism also played a major role in many states, including Florida, where the candidates fought over who would be tougher on combating terrorism.