Republican Rep. Paul Cook of California will not seek reelection in 2020, joining a parade of GOP retirements that has rattled the party over the last few weeks.

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Cook, who represents one of California’s most conservative congressional districts, will instead run for a seat on the San Bernardino County Board of Supervisors, his chief of staff said. The news was first reported by the Los Angeles Times.

The announcement comes the same day that President Donald Trump is making a fundraising swing through the state, with stops scheduled in Palo Alto, Los Angeles and San Diego.

Cook, a fourth-term lawmaker who serves on the House Armed Services and Natural Resources committees, was backed by Trump in his race last year against former Assemblyman Tim Donnelly, a tea party favorite and co-founder of an anti-illegal immigration group that monitors the border

The GOP’s once powerful California delegation has seen its ranks decimated in recent years. Democrats flipped seven seats in the 2018 midterm elections, including in former GOP-strongholds like Orange County, leaving just seven districts represented by Republicans.

While Cook’s seat will likely stay in GOP hands because of its conservative lean, the growing list of retiring Republicans — which now tops over a dozen, including some in key battleground states like Texas — is fueling speculation that GOP lawmakers are growing pessimistic about their chances of winning back the House.

The opening seat immediately drew an establishment-backed Republican contender: Assemblyman Jay Obernolte, who has elevated his standing as his caucus’ point person on budget debates in Sacramento. Obernolte quickly rolled out a website announcing his run for Congress and touting the endorsements of a range of elected California Republicans, including Cook himself.

Donnelly, the hard-line conservative who challenged Cook last cycle, is also another possible contender. Trump’s decision to endorse Cook over Donnelly angered some conservative activists.

While the district has long been Republican territory, Democrats are hoping they can add it to their list of competitive House races given that the GOP has a relatively slim four-point registration margin there.

Democrat Chris Bubser had already launched a campaign for the seat and outraised Cook in the second quarter of this year.