25th District: Polls say California's 25th congressional district race between Democratic challenger Katie Hill and Republican incumbent Steve Knight is a toss-up.
The district includes parts of Los Angeles and Ventura counties, and cities like Santa Clarita, Palmdale and Lancaster.
Congressman Steve Knight in tight race with first-time candidate Katie Hill in Northern LA's 25th District
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 has forecast the race between Republican Young Kim and Democrat Gil Cisneros as a toss-up.
Lottery winner running for Congress in OC
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.
In a New York Times Upshot Siena College poll from late September, Porter leads Walters 48 percent to 43 percent with 8 percent undecided. Porter says a vote for Walters is a vote for President Donald Trump's agenda.
ELECTION 2018: Mimi Walters and Katie Porter face off in OC's 45th District
48th District: Democrats have marked 15-term incumbent Dana Rohrabacher, who represents a large section of Orange County, as the most vulnerable Republican in the country.
Rohrabacher's challenger, businessman Harley Rouda, is a former Republican who has never served in Washington. In a New York Times Siena College poll from early September, Rohrabacher and Rouda are tied at 45 percent.
Incumbent Rohrabacher faces tough challenge from Democrat Rouda