Washington DC - Republicans wrested the US Senate from Democratic control, ending four years in the minority and providing a huge boost to the party in its quest to dominate every branch of government.

While all eyes are on the White House race between Democratic Vice President Kamala Harris and her Republican rival Donald Trump, hundreds of congressional elections will determine how much of the next president's agenda gets enacted.

Jim Justice, the sitting Republican governor of West Virginia, emerged as an easy victor over his Democratic challenger in the Senate race to replace the retiring Joe Manchin, an independent who voted with the Democrats.

Ohio then moved into the Republican column after longstanding Democratic Senator Sherrod Brown was defeated by Bernie Moreno, a Trump-endorsed businessman and the son of a one-time high-ranking Colombian government official.

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Fox News and ABC called the race for control of the upper house after Republican Senator Deb Fischer fended off an unexpectedly robust challenge from independent Dan Osborn in Nebraska.

The Justice and Moreno victories reversed the Democrats' 51-49 Senate advantage, with Republicans looking to extend their lead even further with potential pick-ups in Montana, Wisconsin and Pennsylvania.

Democrats were initially looking to mitigate losses with gains in Texas and Florida, but conceded both as the sitting Republicans – Ted Cruz and Rick Scott – notched easy wins.

If Republicans win all the toss-up races, they'll have 55 of the 100 seats, giving them huge power to usher through Trump's domestic agenda and judicial appointments, should he prevail against Harris.

For the first time in history, two Black women will serve at the same time in the US Senate, following victories from Democrats Angela Alsobrooks and Lisa Blunt Rochester in Maryland and Delaware respectively.

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Of the 2,000-plus Americans who have served in the upper chamber, only three have been Black women – including Harris.

Nonpartisan political finance monitor OpenSecrets reports that $10 billion has been spent on candidates for Congress this cycle – a touch less than in 2020, but almost twice as much as the $5.5 billion price tag for the 2024 White House race.

While the Senate approves treaties and certain presidential appointments, such as ambassadors and Supreme Court nominees, all bills that raise money must start in the House, where the majority could take days to be decided.

The Democrats are in the minority, but overall control looks like a more realistic goal in the lower chamber, where they only need to flip four seats.

"The race for control of the US House remains as close as it's ever been," said the Cook Political Report.

Sarah McBride will be the first openly transgender politician elected to Congress after beating Republican John Whalen III to take a House seat representing Delaware.