Republicans took a step toward winning control of the US Senate with a victory in West Virginia on Tuesday, while neither party appeared to have an edge in the battle for the House of Representatives.
With both chambers of Congress at stake, the results will determine how successfully the next president - Republican Donald Trump or Democrat Kamala Harris - will be able to govern over the next two years.
Republican Jim Justice was projected to win an open seat in West Virginia shortly after polls closed, which would put the party at 50-50 parity with Democrats in the 100-seat chamber. The seat was previously held by Joe Manchin, a Democrat-turned-independent.
The balance of power could shift over the night as results from other races come in.
Nonpartisan analysts say Republicans stand a good chance of taking back the Senate, where Democrats currently hold a 51-49 majority. But Republicans could also lose their grip on the House, where Democrats only need to pick up four seats to take back control of the 435-seat chamber.
As in the presidential election, the outcome will likely be determined by a small slice of voters. The battle for the Senate hinges on seven contests, while fewer than 40 House races are seen as truly competitive.
Democrats are playing defence as they try to retain their hold on the Senate, whose members serve six-year terms. Republicans only need to gain two seats to win control of the chamber, and Justice's victory in West Virginia got them halfway there.
Republicans could secure their majority with a victory in Montana, where Democrat Jon Tester faces a difficult reelection battle, or Ohio, where Democrat Sherrod Brown likewise is locked in a close race.
Republicans stand a chance to widen their Senate majority further if they win races in several competitive Midwestern states. That would allow them to block many of Harris' initiatives and personnel appointments if she were to win the White House, or help Trump deliver on his promised tax cuts if he wins. But they are unlikely to end up with the 60-vote majority needed to advance most legislation in the chamber.
In Nebraska, Republican Senator Deb Fischer faces a surprisingly strong challenge from an independent candidate, Dan Osborn, who has not said whether he would line up with Democrats in the Senate if he were to win.
HOUSE UP FOR GRABS
The picture is less clear in the House, where Republicans hold a narrow 220-212 majority. Analysts say Democrats could easily pick up enough seats to win control of the chamber, though there are no signs of a "wave" election, akin to 2018 or 2010, that would result in a decisive shift in power.
With at least 200 seats safe for each party, the winning side will likely end up with a narrow majority that could make governing difficult. That has been evident in the past two years as Republican infighting has led to failed votes and leadership turmoil and undercut the party's efforts to cut spending and tighten immigration.
Tight races in the heavily Democratic states of New York and California could determine House control, though the final outcome may not be known for several days as California typically takes several days to count its ballots.
Two races in Virginia could give an early indication of how the battle might play out. A Republican victory in the 7th Congressional District, which includes a swath of Washington exurbs, could be a sign that the party has retained its appeal in competitive areas despite its struggles over the past two years. A Democratic victory in the 2nd District, centred on Virginia Beach, could signal that the party is poised for significant gains.
North Carolina's rural 1st District, currently held by Democrat Don Davis, could also serve as a bellwether race, she said.
Voters in Delaware are expected to elect the first transgender member of Congress, as Democrat Sarah McBride is poised to win the state's sole House seat.
The Senate could also see two Black women serving simultaneously for the first time, if Democrat Angela Alsobrooks wins a competitive race in Maryland and Democrat Lisa Blunt Rochester wins as expected in Delaware.
