Republicans have seized control of the U.S. Senate after flipping multiple Democrat-held seats while seeing their own incumbents successfully fend off challenges.

The unexpected battleground of Nebraska pushed Republicans over the top late Tuesday, when incumbent Sen. Deb Fischer brushed back a surprisingly strong challenge from independent newcomer Dan Osborn.

Democrats watched their efforts to salvage their slim majority slip out of reach as tallies rolled in across a map that favoured Republicans, who had to defend just 11 seats in comparison with 21 for the Democrats.

Early in the night, Republicans flipped one seat in West Virginia with the election of Jim Justice, who easily replaced retiring Sen. Joe Manchin, and then did so with seats in Ohio and Montana.

Kentucky Sen. Mitch McConnell, who has been both majority and minority leader for the Republicans since 2007, called it “a hell of a good day.

There have never been more than 55 Republicans in the Senate in a single legislative session, a mark that was still possible for the party to equal.

“I think we were successful before with a narrow majority and I think we will be again,” said McConnell.

The majority leader role will pass from Democrat Chuck Schumer of New York to a Republican in 2025, but it won’t be McConnell, who has said he would serve out his term that ends in 2027 in a different role.

McConnell said Wednesday he would concentrate on advising the White House on foreign policy and defence, calling it the “most dangerous time since right before World War Two,” citing the increased co-operation between Russia, China, North Korea and Iran.

With Donald Trump headed to the White House in a historic comeback, Republicans in the Senate are poised to boost his agenda, though Trump scored relatively few major legislative victories in his first term in office. Senators vote on several White House and government appointees who require confirmation, and they also vote to confirm federal court judges and Supreme Court justices.

In Ohio, three-term Democratic incumbent Sherrod Brown lost his re-election to Colombia-born Republican Bernie Moreno, a wealthy Trump-era newcomer.

Early Wednesday, the Republicans padded their Senate lead when former U.S. Navy SEAL Tim Sheehy defeated three-term Democratic Sen. Jon Tester.

Billions of dollars have been spent by the parties, and outside groups, on the narrow battleground for both the 435-member House and 100-member Senate. The Ohio race between Brown and Moreno was considered the most expensive race of the cycle, with some $400 million US spent.

Republican margin yet to be determined

Republicans were projected to have at least 52 seats by Thursday, to 44 Democrat-aligned senators. Independents Bernie Sanders and Angus King, who each join Democrats in caucus, were re-elected in Vermont and Maine, respectively.

Later Wednesday, The Associated Press and major U.S. networks declared Democrat Elissa Slotkin the winner in Michigan, in the race she faced against Mike Rogers, her Republican opponent.

The AP also said that Democrat Tammy Baldwin, a two-term Wisconsin incumbent, had won her own race over Eric Hovde, a multimillionaire businessman who poured millions of his own money into the campaign.

Senate races that remained too close to call on Wednesday evening included those in Arizona, Nevada and Pennsylvania, all seats Democrats held heading into the election.

While Texas hasn’t elected a Democrat statewide in almost 30 years, Colin Allred, a Dallas-area congressman and former NFL linebacker, positioned himself as a moderate with concerns about the administration’s border policies.

Josh Hawley of Missouri, Roger Wicker of Mississippi and Marsha Blackburn of Tennessee were among the other Republican incumbents who were not greatly troubled by challengers.

Democrats held on to seats that were vacated due to retirements in Delaware and Maryland, sending Lisa Blunt Rochester and Angela Alsobrooks, respectively, to Capitol Hill.

In New Jersey, Andy Kim became the first Korean American elected to the Senate, defeating Republican businessman Curtis Bashaw. The seat opened when Bob Menendez resigned this year after his federal conviction on bribery charges.

The other new senator for the Democrats will be Adam Schiff, a thorn in Trump’s side while a House member. Schiff defeated baseball legend Steve Garvey in California.

New senators for the Republicans in addition to Justice will include Indiana’s Jim Banks and John Curtis of Utah — like Schiff, they are moving to the chamber after serving in the House. Curtis succeeds Mitt Romney, the former presidential candidate and frequent Trump critic.

Republicans will also retain their seat held by J.D. Vance, Trump’s running mate. Ohio’s Republican Gov. Mike DeWine will eventually appoint a replacement for Vance, a person who will serve until a 2026 election is held for the customary six-year Senate term.

Democrats look to prevent sweep

In the House, both Republican Speaker Mike Johnson and Democratic minority leader Hakeem Jeffries expressed confidence heading into the night.

“This historic election has proven that a majority of Americans are eager for secure borders, lower costs, peace through strength and a return to common sense,” Johnson posted on X on Wednesday.

Vote counting in some races could extend for days. California, which has 52 House seats and encourages mail-in voting, has dayslong deadlines where voters are able to “cure,” or fix ballots that have been flagged for ambiguities.

History was made in the House on Tuesday night when Sarah McBride, a Democratic state lawmaker from Delaware who is close to the Biden family, won her race, becoming the first openly transgender person elected to Congress.

As of midday Thursday, The Associated Press had projected 207 wins for the Republicans and 193 for the Democrats, with 38 yet to be called. The party that wins at least 218 seats will take control of the chamber, and members of both parties would vote whether to grant Johnson another term or opt for another Congress member.

Democrats, when they controlled the House from 2019 to 2021, twice impeached Trump, though convictions in the Senate did not occur.