WASHINGTON — Republicans flipped one Senate seat to deadlock the chamber Tuesday, while House races unfolded in a state-by-state slog and polls closed in key states that could decide control of Congress.

Early in the evening, West Virginia Republican Jim Justice won the Senate seat opened by Sen. Joe Manchin's retirement, and Republicans quickly dispatched a late challenge by Democrats in Florida as Republican Sen. Rick Scott sailed to reelection after pouring millions of dollars of his own wealth into the campaign.

In Ohio and the Democratic ''blue-wall'' states of Pennsylvania, Michigan and Wisconsin, Democrats fought to salvage what's left of their slim hold on the Senate.

The Ohio race between Sen. Sherrod Brown and wealthy Trump-backed Bernie Moreno is the most expensive of the cycle, at some $400 million.

And in Nebraska, attention turned suddenly to a state that vaulted to importance with competitive races in both the House and the Senate, where independent newcomer Dan Osborn challenged incumbent GOP Sen. Deb Fischer.

With control of Congress at stake, the ever-tight contests for the House and Senate will determine which party holds the majority and the power to boost or block a president's agenda, or if the White House confronts a divided Capitol Hill.

In the end, just a handful of seats, or as little as one, could tip the balance in either chamber. With a 50-50 Senate, the party in the White House determines the majority, since the vice president is a tie-breaker.

Already several states will send history-makers to the Senate.

Voters elected two Black women to the Senate, Democrat Lisa Blunt Rochester of Delaware and Democrat Angela Alsobrooks of Maryland, in a historic first.

Blunt Rochester won the open seat in her state while Alsobrooks defeated Maryland's popular former governor, Larry Hogan. 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, defeating Republican businessman Curtis Bashaw. The seat opened when Bob Menendez resigned this year after his federal conviction on bribery charges.

Elsewhere, House candidate 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.

The key contests are playing out alongside the first presidential election since the Jan. 6, 2021, attack on the Capitol, but also in unexpected corners of the country after what has been one of the most chaotic congressional sessions in modern times.

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 110,000 voters nationwide, found a country mired in negativity and desperate for change as Americans faced a stark choice between former President Donald Trump and Vice President Kamala Harris.

Congress plays a role in upholding the American tradition of peacefully transferring presidential power. Four years ago, Trump sent his mob of supporters to ''fight like hell'' at the Capitol, and many Republicans in Congress voted to block President Joe Biden's election. Congress will again be called upon to certify the results of the presidential election in 2025.

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.

Top 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 with star lawmakers who helped deliver the party to power.

Other House races are scattered around the country in a sign of how narrow the field has become. Only a couple of dozen seats are being seriously challenged, with some of the most contentious in Maine, the ''blue dot'' around Omaha, Nebraska, and in Alaska.

Vote counting in some races could extend well past Tuesday.

''We're in striking distance in terms of taking back the House,'' House Democratic leader Hakeem Jeffries, who is in line to make history as the first Black speaker if his party wins control, told The Associated Press during a recent campaign swing through Southern California.

But House Speaker Mike Johnson, drawing closer to Trump, predicts Republicans will keep ''and grow'' the majority. He took over after Kevin McCarthy was booted from the speaker's office.

One of the most-watched Senate races, in Montana, may be among the last to be decided. Democrat Jon Tester, a popular three-term senator and ''dirt farmer'' is in the fight of his political career against Trump-backed Tim Sheehy, a wealthy former NAVY Seal, who made derogatory comments about Native Americans, a key constituency in the Western state.

Outgoing Senate Republican leader Mitch McConnell has spent a career focused on seizing and keeping majority power, but other opportunities for Republicans are slipping into long shots.

In the Southwestern states, Arizona firebrand Republican Kari Lake has struggled against Democrat Ruben Gallego in the seat opened by Sen. Krysten Sinema's retirement. In Nevada, Democratic Sen. Jacky Rosen has been holding out against newcomer Sam Brown.

Democrats intensified their challenges to a pair of Republican senators — Ted Cruz of Texas and Scott in Florida — in states where reproductive rights have been a focus in the aftermath of the Supreme Court decision rolling back abortion access. Cruz faces Democrat Colin Allred, the Dallas-area congressman. Scott defeated Democrat Debbie Mucarsel-Powell, a former member of Congress.

What started as a lackluster race for control of Congress was instantly transformed once Harris stepped in for Biden at the top of the ticket, energizing Democrats with massive fundraising and volunteers that lawmakers said reminded them of the Obama-era enthusiasm of 2008.

Fallout from redistricting, when states redraw their maps for congressional districts, is also shifting the balance of power within the House, with Republicans set to gain several seats from Democrats in North Carolina and Democrats picking up a second Black-majority seat in Republican-heavy Alabama.

Lawmakers in the House face voters every two years, while senators serve longer six-year terms.

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.

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Associated Press writers Stephen Groves, Kevin Freking and Farnoush Amiri contributed to this report.