Heck also wrote that the process of compiling the intelligence committee's recent impeachment report had left him "discouraged" about continuing to serve in Congress.

"The countless hours I have spent in the investigation of Russian election interference and the impeachment inquiry have rendered my soul weary," Heck wrote. "I will never understand how some of my colleagues, in many ways good people, could ignore or deny the president’s unrelenting attack on a free press, his vicious character assassination of anyone who disagreed with him, and his demonstrably very distant relationship with the truth."

Heck served a decade in the Washington state House, beginning with his election in 1976. He also served as chief of staff to then-Washington Gov. Booth Gardner during Gardner's second term in Olympia. He first ran for Congress in 2010, losing to now-Rep. Jaime Herrera Beutler (R-Wash.). Two years later, he won a different seat created by the post-2010 redistricting.

In Congress, Heck led recruitment efforts for the DCCC for the past two cycles, personally drafting many of the newly elected Democratic freshman members, including Reps. Stephanie Murphy (D-Fla.) and Susie Lee (D-Nev.).

He ran unsuccessfully last year to serve as chairman of the committee, losing a four-way race to Rep. Cheri Bustos (D-Ill.).

Washington's 10th District is Democratic-leaning territory. Heck won reelection last year with 62 percent of the vote, and Hillary Clinton carried the district by an 11-point margin in 2016.

Heck's retirement will likely result in a crowded field of candidates vying to succeed him. Under Washington's top-two primary system, all candidates will run together on the same ballot, regardless of party, in the Aug. 4 primary, with the top two finishers advancing to a head-to-head general election.

Possible Democratic candidates for the seat include Olympia Mayor Cheryl Selby; Phil Gardner, Heck's district director; and state Reps. Beth Doglio, Christine Kilduff and Mari Leavitt. Heck already had a Democratic challenger next year: truck driver Joshua Collins.