Republicans have opportunities to pick up competitive House seats Democrats looking to advance their political careers to the Senate are leaving vacant.

Reps. Katie Porter (D-CA) and Elissa Slotkin (D-MI) announced their candidacies for U.S. Senate in their respective states last month as mainstay Senate Democrats Dianne Feinstein (D-CA) and Debbie Stabenow (D-MI) head for retirement.

But the career-oriented moves are leaving Democrats vulnerable in California’s Forty-Seventh and Michigan’s Seventh Congressional Districts.

Porter and Slotkin are coming off the heels of tough reelection campaigns, where they heavily outspent their Republican counterparts to eke out close victories, as Politico’s Brittany Gibson noted.

Porter, for instance, beat out Republican candidate Scott Baugh in November by a margin of 51.7 percent to 48.3 percent, as the New York Times reported. She relied on a $26 million fundraising machine last year, shelling out $18.5 million in advertising alone, according to Gibson. Conversely, Baugh raised only $3 million, a third of which was spent on advertisements.

In other words, Porter’s more than $23 million advantage translated to a victory of 9,113 votes. With Porter’s strong fundraising apparatus out of play, Republicans could potentially flip the district.

“People just have a lot of assumptions [that] because I was a good fundraiser that it must have been like a nothing burger,” Porter told Politico.

Fred Whitaker serves as the Republican party Chairman in Orange County, California, which is within the 47th District. In January, he said that the seat is the party chapter’s “top target,” as Breitbart News reported.

Reps. Adam Schiff (D-CA) and Barbara Lee (D-CA) have both announced bids for Feinstein’s seat as well, although the seats they leave vacant are in far safer districts. Schiff took nearly three-quarters of the vote in the Thirtieth Congressional District last year, while Lee secured nearly 90 percent in the Twelfth District.

Slotkin leaves a far less safe void for her party. Like Porter, she took 51.7 percent of the vote in her district in the midterm election, besting Republican challenger Tom Barrett, who received 46.3 percent support. A libertarian candidate ran as well, siphoning off two percent of the vote. Slotkin was able to fund $8.6 million in advertisements to Barrett’s nearly $900,000, per Politico.

Thomas Morgan, the Democrats’ district chairman, told Politico he expects an “extremely competitive” race to fill Slotkin’s seat.