WASHINGTON (AP) — Republicans have taken control of the U.S. Senate and are fighting to keep their majority in the U.S. House, which would produce a full sweep of GOP power in Congress alongside President-elect Donald Trump in the White House.

A unified Republican grip on Washington would set the course for Trump’s agenda. Or if Democrats wrest control of the House, it would provide an almost certain backstop, with veto power over the White House.

Trump, speaking early Wednesday at his election night party in Florida, said the results delivered an “unprecedented and powerful mandate” for Republicans.

He called the Senate rout “incredible.” And he praised House Speaker Mike Johnson, who dashed from his own party in Louisiana to join Trump. “He’s doing a terrific job,” Trump said.

Vote counting in some races could go on for days, and control of the House is too early to call.

The rally for Republicans started early on election night in West Virginia, when Jim Justice, the state’s wealthy governor, flipped the seat held by retiring Sen. Joe Manchin. From there, the Republicans marched alongside Trump across the Senate map.

Democratic efforts to oust firebrand Republicans Ted Cruz of Texas and Rick Scott of Florida collapsed. The unexpected battleground of Nebraska pushed Republicans over the top. Incumbent GOP Sen. Deb Fischer brushed back a surprisingly strong challenge from independent newcomer Dan Osborn.

One of the most-watched Senate races, in Montana, was decided early Wednesday. Democrat Jon Tester, a popular three-term senator and “dirt farmer” in the fight of his political career lost to Trump-backed Tim Sheehy, a wealthy former NAVY Seal, who made derogatory comments about Native Americans, a key constituency in the Western state.

All told, Senate Republicans have a chance to scoop up more seats, potentially delivering their most robust majority in years — a coda to outgoing GOP Minority Leader Mitch McConnell, who made a career charting a path to power, this time by recruiting high-wealth Republicans aligned with Trump. The Senate races that topped $2 billion.

The fight for control of the House became a state-by-state slog, much of which unfolded far from the presidential race.

House races are focused in New York and California, where Democrats are trying to claw back some of the 10 or so seats where Republicans have made surprising gains in recent years.

Other House races are scattered around the country, with some of the most contentious in Maine, the “blue dot” around Omaha, Nebraska, and in Alaska.

To gain control of the House, Democrats need to flip four seats from Republicans, while holding all of their own, a tall task especially in congressional districts where Trump has won.

It could come down to just a handful of seats, or as little as one, to determine House control.

Harris ignited enthusiasm for her party when Biden dropped out of the race, and she stepped in atop the ticket, a head-spinning development barely 100 days from the election. But Democrats watched their own hopes for a sweep of Washington fizzle.

Voters said the economy and immigration were the top issues facing the country, but the future of democracy was also a leading motivator for many Americans casting ballots in the presidential election.

AP VoteCast, an expansive survey of more than 120,000 voters nationwide, found a country mired in negativity and desperate for change as Americans faced a stark choice between Trump and Vice President Kamala Harris.

This is the first presidential election since the Jan. 6, 2021, attack on the Capitol, when Trump sent a mob of his supporters to “fight like hell” against the 2020 election. Many Republicans in Congress voted against President Joe Biden’s victory. Congress will again be called on next year to certify the 2024 election.

Even still, the election followed one of the most chaotic congressional sessions in modern times as the Republican-led House kicked out its speaker, Kevin McCarthy, threatened government shutdowns and had difficulty conducting the basic operations of governance.

Johnson has said Republicans in the House and Senate have been working on an “ambitious” 100 day-agenda — cutting taxes, securing the U.S. border and taking a ”blowtorch” to federal regulations — if they sweep the White House and Congress.

Trump himself has promised mass deportations and retribution on his perceived enemies. Republicans want to push federal agencies out of Washington and restaff the government workforce, Johnson said, to bring the federal government “to heel.”

Trump is “thinking big” about his legacy, Johnson said.

Several states will send history-makers to the new Congress.

Voters elected two Black women to the Senate, Democrat Lisa Blunt Rochester of Delaware and Democrat Angela Alsobrooks, who defeated Republican Larry Hogan, the former governor, in Maryland. Just three Black women have served in the Senate, and never before have two served at the same time.

And in New Jersey, Andy Kim became the first Korean American elected to the Senate. The seat opened when Bob Menendez resigned this year after his federal conviction on bribery charges.

In the House, candidate Sarah McBride, a Democratic state lawmaker from Delaware who is close to the Biden family, became the first openly transgender person elected to Congress.

What’s still unclear is who will lead the new Republican Senate, as longtime leader McConnell prepares to step down from the post.

South Dakota Sen. John Thune, the No. 2 Republican, and Texas Sen. John Cornyn, who previously held that post, are the front-runners to replace McConnell in a secret-ballot election scheduled for when senators arrive in Washington next week.

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.

If the two chambers do in fact flip party control, as is possible, it would be rare.

Records show that if Democrats take the House and Republicans take the Senate, it would be the first time that the chambers of Congress have both flipped to opposing political parties.