A leading progressive group announced endorsements for Democratic challengers in four California House races that are considered some of the party's top-priority seats to flip in 2018.

Democracy for America is throwing its support behind Democratic candidates running to unseat California GOP Reps. Steve Knight, Dana Rohrabacher Dana Tyrone RohrabacherDemocrat Harley Rouda advances in California House primary Lawyers to seek asylum for Assange in France: report Rohrabacher tells Yahoo he discussed pardon with Assange for proof Russia didn't hack DNC email MORE, Ed Royce Edward (Ed) Randall RoyceThe 'extraordinary rendition' of a US Presidential Medal of Freedom recipient, 'Hotel Rwanda' hero Gil Cisneros to face Young Kim in rematch of 2018 House race in California The most expensive congressional races of the last decade MORE and Darrell Issa Darrell Edward IssaDCCC reserves new ad buys in competitive districts, adds new members to 'Red to Blue' program Wife of former Rep. Duncan Hunter sentenced to 8 months of home confinement Harris endorses Democrat in tight California House race MORE, who all represent districts won by Hillary Clinton Hillary Diane Rodham ClintonButtigieg stands in as Pence for Harris's debate practice Senate GOP sees early Supreme Court vote as political booster shot Poll: 51 percent of voters want to abolish the electoral college MORE in 2016. The group believes focusing on these four races will be essential if Democrats want to regain control of the House next year.

“Victories across California in 2018 are going to be absolutely critical to Democrats’ chances of taking back the House and gaining the political power necessary to confront and defeat Donald Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE’s bigoted and oligarchic agenda,” Democracy for America Chairman Jim Dean said in a statement to The Hill.

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“By sending Bryan Caforio, Laura Oatman, Sam Jammal and Mike Levin to Congress, Californians will have the new warriors they need to fight back and Democracy for America members couldn’t be more excited to stand with them every step of the way,” he said.

Caforio, an attorney, has launched a rematch bid after running against Knight last cycle. Despite Clinton winning the district by more than 6 points, Knight defeated Caforio by nearly the same margin. The race drew heavy outside spending last cycle, which will likely happen again in 2018.

Oatman, who is challenging Rohrabacher, is an architect and business owner. Clinton won the district by nearly 2 points. Rohrabacher, who has been in Congress for nearly three decades and is known for his pro-Russia stance, has drawn a handful of Democratic challengers.

Jammal, a former Obama official, is running to unseat Royce, who’s been in Congress for more than two decades. Clinton won Royce’s district by more than 8 points. But Jammal will also need to compete in a crowded primary for the nomination, and several other major Democratic groups including EMILY’s List and VoteVets have endorsed other challengers in the primary.

Levin is an environmental attorney challenging Issa. Clinton won Issa’s district by more than 7 points. Levin will face Doug Applegate, who lost to Issa by less than a point in 2016, in the primary, as well as Paul Kerr. But Levin and Applegate have already been exchanging sharp attacks at one another.

GOP-held districts like these four in southern California will be critical for Democrats to win as they seek to flip 24 seats to take back the House.

This isn’t the only race in California where Democracy for America plans to be active.

The group also endorsed California Senate Pro Tem President Kevin de León’s (D) bid against longtime Sen. Dianne Feinstein Dianne Emiel FeinsteinBiden leads Trump by 12 points among Catholic voters: poll Names to watch as Trump picks Ginsburg replacement on Supreme Court McConnell says Trump nominee to replace Ginsburg will get Senate vote MORE (D-Calif.). Progressives are looking to unseat Feinstein in 2018, arguing she hasn’t held Trump accountable.

But a recent poll shows she has a comfortable lead over de León.