Democratic Rep. Katie Hill’s decision to resign amid allegations of an improper relationship with her staffer has upended the battle to control a competitive district in Southern California.

Republicans believe they can win the 25th District back, especially now that they don’t have to face Hill and her sizable war chest, which was already at $1.5 million on Sept. 30. But Democrats contend that the district will stay in their hands, citing partisan dynamics and demographics of the district.

It’s unclear when exactly Hill will resign, which has cast a layer of uncertainty into the race. Democratic Gov. Gavin Newsom can’t call for a special election until after the vacancy takes effect. But both parties are gearing up for a competitive — and expensive — battle.

Moving left?

The 25th District north of Los Angeles, which includes former President Ronald Reagan’s presidential library, had long been GOP territory. But Hill defeated incumbent GOP Rep. Steve Knight by 9 points in 2018, and Hillary Clinton carried the district by 7 points in 2016. Republicans have been in decline throughout California, and the GOP lost seven House seats in 2018, including the 25th District.

The partisan dynamics of the district and the national environment appear to favor Democrats, and Inside Elections with Nathan L. Gonzales is keeping the 25th District rating at Likely Democrat despite Hill’s impending resignation.