If decorated combat veteran Mast leaves office, two Republicans would be left to contend for the 2018 nomination to represent the District 18 seat: physician Mark Freeman and businessman Dave Cummings. The deadline for congressional candidates to qualify for the 2018 ballot passed last week.
Two Democrats are running for Mast's District 18 seat: Lauren Baer, a former Obama administration foreign policy adviser, and Pam Keith, a Navy veteran and attorney.
The Washington Democratic establishment has anointed Baer as its choice to topple Mast. She raised more than $1 million through March with fundraising help from House Democratic Leader Nancy Pelosi and has campaigned with House Democratic Whip Steny Hoyer. Keith, who got a surprisingly strong 15.4 percent in a 2016 Democratic primary for U.S. Senate, is touting her grassroots support in the District 18 Democratic primary.