Control of US Congress at stake in tight election battle - explainer

Control of the US Congress is at stake on Tuesday in elections that could flip both the House of Representatives and the Senate, while still leaving Capitol Hill divided between Donald Trump's Republicans and Kamala Harris' Democrats.

The outcome will play an important role in determining how easily the winner of Tuesday's US presidential election will govern until the next congressional elections in 2026.

Nonpartisan analysts say Republicans stand a good chance of taking back the Senate, where Democrats 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.

"It's incredibly close," said Erin Covey, who analyzes House races for the nonpartisan Cook Political Report.

Voters do not seem to be indicating a clear preference for either party. An October Reuters/Ipsos poll found 43% of registered voters would back the Republican candidate in their district, while 43% would back the Democratic candidate.

Democrats are playing defense 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 they are expected to easily pick up one of those seats with a victory in West Virginia, where Joe Manchin, a Democrat-turned-independent, is retiring. The state's popular governor, Jim Justice, is forecast to easily capture Manchin's seat.

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, which can take days to count ballots, and recounts and runoffs of close races can take weeks to resolve.

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, centered on Virginia Beach, could signal that the party is poised for significant gains, the Cook Political Report's Covey said.

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. McBride is running for a seat vacated by Democrat Lisa Blunt Rochester, who is running for a Senate seat.

Blunt Rochester could also make history -- if she wins her race and fellow Democrat Angela Alsobrooks also wins a more competitive US Senate election in neighboring Maryland, they would be the first two Black women to serve simultaneously in that chamber.