LOS ANGELES — It seemed that things could hardly get much worse for California Republicans.

But on Thursday, the party conceded a seventh congressional seat in California, as Democrats completed a clean sweep of all the Republican-held seats in California districts where Hillary Clinton had defeated President Trump. Democrats are now likely to take 40 seats from Republicans as they decisively capture control of Congress.

David Valadao, a Republican who has represented Central California since 2013 — and won re-election in 2016 with a sturdy 57 percent of the vote — was defeated by T.J. Cox, an engineer making a second bid for Congress. “Representing the Central Valley in Congress has been the honor of a lifetime,” Mr. Valadao said in his concession statement.

Mr. Valadao’s loss was a particularly dispiriting turn of events for Republicans because he had initially been declared the winner of the race by The Associated Press. And from the start, he seemed the best situated of the seven Republicans to withstand challenges by Democrats, given his previous strong winning margins.