US Senator Cortez Masto, the first Latina elected to the Senate, will keep her Nevada seat after defeating Republican Adam Laxalt.
Democrats now hold 50 seats and Republicans 49, with Georgia still to declare.
However, Democrats will be able to hold a majority with Vice President Kamala Harris’s casting vote.
It comes after mail-in ballots from Clark Country, which includes Las Vegas, overwhelmingly voted for it.
Donald Trump endorsed her opponent Mr Laxalt, although many of the former president’s nominees in the midterms have performed badly
Mr Trump has called the results of the midterm elections “somewhat disappointing” on his social media platform Truth.
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