WASHINGTON —  Republicans picked up a crucial win Tuesday in the race for the Senate majority, as Jim Justice easily notched the West Virginia seat to succeed retiring Sen. Joe Manchin III, deadlocking the chamber in a 50-50 split, for now.

Justice, the state’s governor who often appears with his English bulldog, Babydog, was widely expected to deliver for Republicans as they work to wrest control from Democrats. Republican Donald Trump is popular in the state, and Manchin, who left the Democratic Party to become an independent, declined to seek another term.

West Virginia is the first of several states where Democrats see their slim hold on the chamber at serious risk. In a 50-50 split, the majority goes to the party in the White House, because the vice president can cast tie-breaking votes. There are more races ahead.

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, only a handful of seats, or even just one, could tip the balance in either chamber.

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 as Americans faced a stark choice between former President 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 Biden’s election. Congress will again be called upon to certify the results of the presidential election in 2025.

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, Neb.; 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 of New York, 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 “grow” their majority. He took over after Kevin McCarthy of Bakersfield was booted from the speaker’s office.

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.

Republicans put the Senate Democrats, with just slim control of the chamber, on defense across a wide map in several states favorable to the GOP.

In Ohio, Trump-backed Republican Bernie Moreno, a Cleveland businessman, is seeking to unseat three-term Democratic Sen. Sherrod Brown. Some $400 million has been spent on the race.

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.

And across the “blue wall” battlegrounds of Pennsylvania, Michigan and Wisconsin, Republicans are depending on Trump as they try to unseat a trio of incumbent Democratic senators.

Outgoing Senate Republican leader Mitch McConnell of Kentucky 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 independent Sen. Kyrsten Sinema’s retirement. In Nevada, Democratic Sen. Jacky Rosen has been holding out against Republican newcomer Sam Brown.

Democrats intensified their challenges to a pair of Republican senators — Ted Cruz of Texas and Rick 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, while Scott has poured $10 million of his own fortune into the race against Debbie Mucarsel-Powell, a former House lawmaker.

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.

Congress has a chance to reach several history-making milestones as it is reshaped by the American electorate and becomes more representative of a diverse nation.

Not one, but possibly two Black women could be on their way to the Senate, which would be something never seen in the U.S.

Democrat Lisa Blunt Rochester of Delaware is favored in the Senate race against Republican Eric Hansen.

And in Maryland, Harris ally Angela Alsobrooks is in a highly competitive race against the state’s popular former governor, Republican Larry Hogan.

Americans have elected two Black women, including Harris, as senators since the nation’s founding, but never at the same time.

House candidate Sarah McBride, a state lawmaker from Delaware who is close to the Biden family, is poised to become the first out transgender person in Congress.

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.

Mascaro and Jalonick write for the Associated Press. AP writers Stephen Groves, Kevin Freking and Farnoush Amiri contributed to this report.