California Democrats completed their sweep of the congressional delegation in Orange County on Saturday as Gil Cisneros defeated Young Kim, a Republican, to capture a seat in what had once been one of the most conservative Republican bastions in the nation.

The victory by Mr. Cisneros, a philanthropist, was declared by The Associated Press. It completes what has amounted to a Democratic rout in California this year. Democrats set out to capture seven Republican-held seats where Hillary Clinton defeated President Trump in 2016, including four in Orange County. They won six of them.

Representative David Valadao, from the Central Valley, is the only Republican who survived the Democratic onslaught in those seven districts, according to The Associated Press. His margin has shrunk as mail-in votes have continued to be counted. The deadline for counting those votes in California is Dec. 7.

With Mr. Cisneros’s victory, Democrats now control all the House seats that represent Orange County — the birthplace of Richard M. Nixon and modern-day conservatism. The party also won supermajorities in the California Assembly and Senate, while the party’s candidate for governor — Gavin Newsom, the lieutenant governor — easily turned back a Republican challenge. Democrats control every statewide elected position in California.