Representative Dana Rohrabacher, a Republican fixture in California who represented Orange County for 15 terms, has lost his bid for re-election. His defeat underlines the party’s setbacks in a part of the state that was long a symbol of its political dominance.
The Associated Press called the race on Saturday, with Harley Rouda receiving 52 percent of the vote to Mr. Rohrabacher’s 48 percent.
Mr. Rouda, 56, is a former Republican turned Democrat who became a symbol of the Democratic efforts to win back Congress this year. Mr. Rohrabacher, 71, was viewed as particularly vulnerable because he defended Russia in the midst of allegations about its efforts to intervene in elections in the United States.
In July, Mr. Rohrabacher admitted to meeting with Maria Butina, who was charged by federal prosecutors this summer with conspiracy and acting as a foreign agent, during his trip to Russia in 2015. Mr. Rohrabacher was considered so valuable to the Kremlin that the F.B.I. warned him in 2012 that Russia regarded him as an intelligence source worthy of a code name. His support for President Vladimir Putin of Russia raised questions even among fellow Republicans.
Mr. Rouda capitalized on that in his campaign, running ads emphasizing Mr. Rohrabacher’s Putin ties and criticizing Mr. Rohrabacher for questioning the assertion by United States intelligence agencies that Russia interfered in the 2016 election.
Mr. Rouda has said he is in favor of Medicare for All and a $15-an-hour federal minimum wage tied to inflation, along with free tuition at public colleges. In a post on Twitter after the race was called, he thanked his supporters “who dedicated countless hours to flipping” the 48th Congressional District. He told The Associated Press that he intended to be “honest, transparent, accessible and tireless” in Washington.
Democrats have focused on seven seats in California that were controlled by Republicans. The loss by Mr. Rohrabacher marks the third Democratic victory in those races.
On Saturday night, three other races remained too close to call.
Josh Harder, a Democrat, was ahead of Representative Jeff Denham by 1.9 percentage points in the 10th District. Representative Mimi Walters, a Republican, was leading Katie Porter by 0.9 percentage point in the 45th District. And Young Kim, a Republican, was beating Gil Cisneros by 1.4 percentage points in a race to replace Representative Ed Royce, a Republican who is retiring from the 39th District seat.
Support for Republicans has been steadily eroding in Orange County; in 2016, Hillary Clinton became the first Democratic presidential candidate to win there since the Great Depression. Roughly a third of voters in the county are registered Republicans, down from more than half in 1990. About 30 percent of voters in the district are not registered with either party.