Dan Koh, who worked as a White House aide during the presidency of Joseph R. Biden Jr., won the Democratic primary in a solidly blue Massachusetts congressional district on Tuesday, according to The Associated Press, topping a crowded field that pursued an open seat.
In a costly race in the state’s Sixth Congressional District, Mr. Koh defeated rivals including Tram Nguyen, a progressive state representative who as a child fled Vietnam with her family as political refugees, and John Beccia, a largely self-funded businessman who ran as an outsider.
Mr. Koh is the only Democratic congressional candidate this year endorsed by both Mr. Biden and former Vice President Kamala Harris. While many Democrats are seeking to turn the page on the Biden administration and the party’s 2024 election defeat, Mr. Koh highlighted his connections.
“You will be one heck of a congressman,” Biden said in a phone call that was released by Mr. Koh’s campaign in May. “Everything you and I fought for, everything we did to help ordinary people, is in jeopardy now in this administration, so we got to get you elected.”
The race in the Sixth Congressional District, which spans the state’s North Shore, opened up after Representative Seth Moulton, a moderate Democrat, declined to seek re-election and instead challenged Senator Ed Markey. Mr. Moulton lost his primary on Tuesday night.
The district leans heavily Democratic, and Mr. Koh will be a clear favorite against Micah Jones, an Army veteran and lawyer who ran uncontested in the Republican primary.
If Mr. Koh wins the general election in November, he would be the first Asian American person to represent Massachusetts in Congress.
Mr. Koh also has the endorsement of Marty Walsh, a former mayor of Boston and a labor secretary under Mr. Biden. Mr. Koh was Mr. Walsh’s chief of staff at the Labor Department before taking a job at the White House as deputy cabinet secretary.
Mr. Koh’s competitors sought to use his résumé against him, negatively labeling him as an insider. But his connections powered strong fund-raising and allowed him to outspend Ms. Nguyen by a wide margin on advertising in the race. Mr. Koh spent $2.4 million and Ms. Nguyen $158,000 on television, radio and digital ads, according to data from AdImpact. Mr. Beccia spent $3.2 million on ads.
Mr. Koh defended Mr. Biden during the primary, saying in a television appearance in May 2025 that he “was never concerned about his ability to make the right decision, or his team around him making the right decision, for the country.”
Mr. Koh was one of just two U.S. House candidates whom Mr. Biden endorsed this cycle. (The other House candidate Mr. Biden endorsed, Nikki Gronli, worked for the Department of Agriculture in his administration. She won the Democratic primary for South Dakota’s at-large congressional seat in June.)
