Below are results coming in from the key congressional races:
48th District: Democrats marked 15-term incumbent Dana Rohrabacher, who represents a large section of Orange County, as the most vulnerable Republican in the country. As of Wednesday afternoon, it appears Rohrabacher may lose his seat to political newcomer and opponent Harley Rouda -- though a winner has not officially been announced.
Rouda is a former Republican who has never served in Washington.
25th District: Polls said California's 25th congressional district race between Democratic challenger Katie Hill and Republican incumbent Steve Knight was a toss-up.
As of Wednesday afternoon, Hill stood at more than 51 percent of the votes, while Knight had 48.7 percent. Hill's campaign manager told ABC News that Knight has called and conceded.
The district includes parts of Los Angeles and Ventura counties, and cities like Santa Clarita, Palmdale and Lancaster.
39th District: California's 39th District, which is mostly in Orange County but includes parts of Los Angeles and San Bernardino counties, is one of the most competitive congressional races in the country.
ABC News' political analysis site FiveThirtyEight forecasted the race between Republican Young Kim and Democrat Gil Cisneros as a toss-up.
45th District: Republican congresswoman Mimi Walters is in a very tight race against Democrat Katie Porter, a UC-Irvine law professor who has worked extensively in consumer protection.
Porter says a vote for Walters is a vote for President Donald Trump's agenda.