Dan Koh won a crowded Democratic primary Tuesday for the Massachusetts House seat being vacated by Rep. Seth Moulton, defeating a Squad-backed progressive to secure the nomination in a contest heavily influenced by party establishment figures Joe Biden and Kamala Harris.

The victory gives Koh, a former chief of staff to Boston Mayor Marty Walsh, a second chance at Congress after losing a primary for a different district in 2018. His win represents a significant boost for moderate Democrats seeking to counter the influence of the party's progressive wing heading into the midterms.

Biden and Harris Backed Koh

Koh secured endorsements from Biden and Harris during the primary campaign, lending him the weight of the Democratic establishment against a progressive challenger who received support from Squad members and other left-wing activists. The primary became a proxy battle between the moderate and progressive factions of the party, with Koh's victory signaling continued strength for the establishment wing.

The Massachusetts seat has been held by Moulton for several terms before he announced his decision not to seek reelection. The district is safely Democratic, meaning Koh is heavily favored to win the general election in November and take his seat in the House in January.

Squad Influence Tested

The defeat of the Squad-backed candidate raises questions about the electoral strength of the progressive movement in Democratic primaries outside of heavily left-leaning districts. While Squad members like Rep. Alexandria Ocasio-Cortez and Rep. Ilhan Omar have successfully defended their own seats and helped elect allies in similar urban areas, their ability to expand the movement into more moderate districts remains limited.

Squad-aligned groups have won several Democratic primaries in the 2026 cycle, including seats in Texas, Pennsylvania, New Jersey, New York, Colorado, and Michigan. However, those victories have largely come in districts with progressive voting bases rather than in competitive primaries where moderate candidates have establishment backing.

Moderate Wing Gains Ground

Koh's victory adds to a string of wins for moderate and establishment-backed Democrats over progressive challengers this cycle. The results suggest that while the progressive movement has grown in influence within the Democratic Party, it has not eclipsed the power of traditional party leaders and organizations when they throw their support behind a preferred candidate.

The primary also highlights the role of money and endorsements in shaping outcomes. Koh benefited from fundraising advantages and the organizational muscle of Democratic establishment networks, advantages that progressive candidates often struggle to match without equivalent institutional support from labor unions or grassroots donors.

With Koh's nomination secured, Democrats can turn their attention to the general election, where the party is expected to hold the seat comfortably. His path to Congress marks a comeback from his 2018 loss and positions him as a potential ally of the moderate wing in debates over the party's direction in the House.