The political fight over Orange County isn't over.

Republican candidates are starting to line up with plans to take back four U.S. House seats all or partly in the California county that Democrats seized last year on their way to retaking control of the chamber.

Orange County, a neighbor to the southeast of heavily Democratic Los Angeles County, was once a nationally known Republican stronghold. But the population diversified, changing its politics. It is now roughly divided between Republicans and Democrats.

Hillary Clinton won the county in 2016 before Democrats grabbed a cluster of GOP-held House seats last year.

The latest sign of the 2020 campaign to come: Republican Young Kim filed federal paperwork to run in the state's 39th Congressional District that cuts across the northern part of the county. Her candidacy could set up a potential rematch with Democrat Gil Cisneros, who defeated her in 2018.

Republican National Committeeman Shawn Steel, who lives in the county, said the GOP is eager to reverse the 2018 loses. He said members of Congress in their first terms, like Cisneros, remain vulnerable.

He attributed the 2018 losses to a financial advantage for Democratic candidates that he predicted wouldn't happen again.

"You didn't have a blue wave, you had a green wave," he said.

Earlier this month, Orange County prosecutor Ray Gennawey, a Republican, announced his campaign in the county's 45th District, where Democrat Katie Porter defeated Republican Rep. Mimi Walters last year.

First-term Democratic Reps. Harley Rouda in the 48th District and Mike Levin in the 49th will also be defending seats all or partly in Orange County.