Fox News Channel is projecting that the Democrats will win control of the US House of Representatives.
It did not project the margin of victory or elaborate immediately on the basis for its projection.
Fox was alone among US media outlets with the call as Americans continued to cast their ballots on Tuesday.
Democrats have been widely expected to pick up the seats needed to wrest control of the chamber from President Donald Trump's Republicans, who were expected to maintain control of the Senate.
Other media outlets said the outcome in the House was leaning in Democrats' favour, but that it was too soon to project a Democratic victory.
