The Republican Party has made gains in the Senate and secured a majority for the first time in four years.
A third of the Senate’s 100 seats were up for grabs at this year’s elections, with 51 required for a majority.
The Senate, similar to the House of Lords in the UK but is elected, is made up of two senators for each state.
The Democrats have been in control of the upper house of Congress for the last four years.
US election latest: Trump wins key battleground states
But the Republicans have wrestled control after making gains in elections overnight.
With eight seats yet to be declared, the Republicans have 51 seats in the Senate and the Democrats have 40.
Independent candidate Bernie Sanders has been projected to win a fourth term representing Vermont.
Among the Republicans’ gains were Trump-backed Bernie Moreno, who flipped a seat in Ohio which had been held by the Democrats since 2007.
Another gain was in West Virginia where Jim Justice won a seat that was left vacant after independent senator and former Democrat Joe Manchin’s retirement.
All 435 seats in the House of Representatives, the lower house, are also up for grabs.
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The Republicans’ gains in the Senate come as Donald Trump is projected to win several key battleground states in the race to the White House, narrowing Kamala Harris’s path to victory.
He is expected to address his supporters in the early hours of Tuesday morning, while Kamala Harris will not speak until later on Wednesday.