Former White House adviser Dan Koh, the only candidate endorsed by both Biden and Harris, wins Massachusetts Democratic House primary

Two endorsements, two victories. This is how former President Joe Biden backed candidates in this cycle's congressional primaries, after his former advisor, Dan Koh, won the Democratic primary for Massachusetts' 6th Congressional District on Tuesday, weeks after Nikki Gronli did the same in South Dakota. They were the only House candidates to receive the former president's endorsement all season.

In addition to Biden, Koh also secured the support of former Vice President Kamala Harris, something no other Democratic congressional candidate achieved this year. In an election cycle in which much of the party chose to distance itself from the previous administration after the 2024 defeat, Koh did the opposite and made that closeness the centerpiece of his campaign.

This became clear in May, when his team released audio from a call with Biden. “You will be one heck of a congressman,” the former president told him. “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.”

To secure the nomination, Koh faced an uphill battle, prevailing in a crowded and expensive race. Among his rivals were Tram Nguyen, a progressive lawmaker who came to the country as a refugee after fleeing Vietnam as a child, and John Beccia, a businessman who ran as an outsider and funded much of his campaign with his own money. The seat became vacant after Representative Seth Moulton decided not to seek reelection to challenge Senator Ed Markey, a challenge he also lost by a wide margin tonight.

Koh's own past as a former advisor brought him as many benefits as it did criticism. His opponents labeled him an establishment figure, but those same contacts gave him a clear financial advantage. In total, he spent $2.4 million on advertising, according to AdImpact, compared to Nguyen's $158,000. Beccia, despite lacking major endorsements, spent $3.2 million out of his own pocket. Marty Walsh, former mayor of Boston and former Secretary of Labor under Biden, also endorsed Koh, for whom he served as chief of staff before joining the White House.

With the nomination settled, Koh is emerging as the clear favorite for November against Republican Micah Jones, an Army veteran who faced no opposition in his own primary, in a district that has historically leaned Democratic. If he wins the general election, he will be the first Asian American to represent Massachusetts in Congress.