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Dan Koh, a former chief of staff to Boston Mayor Marty Walsh, won the Democratic primary for Massachusetts' 6th Congressional District on Tuesday, capping a costly six-way race for the seat U.S. Rep. Seth Moulton is vacating to run for the U.S. Senate.
The Associated Press called the race for Koh at 10:27 p.m. With 63 percent of the vote counted, Koh led Nguyen, of Andover, 42.3 percent to 29.5 percent. Financial-technology executive John Beccia was third with 12.3 percent, followed by former state Rep. Jamie Belsito at 8.6 percent and Mariah Lancaster at 3.7 percent. Lancaster ended their campaign three days before the primary and endorsed Nguyen but remained on the ballot; software engineer Bethany Andres-Beck rounded out the field.
Nguyen conceded to supporters Tuesday night, calling the effort "this incredible people-powered grassroots campaign" despite the result. "We ran against millions of dollars, and look what we accomplished together," she said, thanking volunteers for "every door you've knocked, every text you sent, every single person you talked to."
Beccia conceded in a statement Tuesday night, saying he had called Koh to congratulate him. "I called Dan Koh a few minutes ago to congratulate him on winning tonight, and I wished him well representing our community," he said, adding that the voters he met across the North Shore "hold down two jobs and still fall behind, while our politicians are out of touch."
Koh's win makes him the first Asian American elected to Congress from Massachusetts, pending confirmation against U.S. House historical records.
In his victory speech Tuesday night, Koh traced that history to his family. His mother's side emigrated from a village south of Beirut, Lebanon; his father's mother fled North Korea as a refugee, and his grandfather was exiled after the overthrow of South Korea's democratic government. "I know that tonight, I stand on their shoulders," Koh told supporters.
The seat opened when Moulton, a Salem Democrat who has held it since January 2015, decided not to seek reelection and instead ran in Tuesday's Democratic primary for U.S. Senate, challenging 80-year-old Sen. Edward Markey on a message of generational change. Moulton, a Marine Corps veteran who served four tours in Iraq, won the 6th District seat in 2014 by defeating nine-term incumbent U.S. Rep. John Tierney in the primary — the first sitting Massachusetts congressman to lose a primary since 1992 — then beat Republican Richard Tisei that November.
Money splits a crowded field
Koh, of Andover, is a former chief of staff to Walsh and a Biden White House aide who lost a 2018 congressional primary to now-U.S. Rep. Lori Trahan by 145 votes. Nguyen, an Andover state representative and former Greater Boston Legal Services attorney, came to the United States from Vietnam as a 5-year-old refugee and became the first Vietnamese American woman elected to the Massachusetts Legislature in 2018; she campaigned on a legislative record that includes a coercive-control law she co-wrote with state Rep. Natalie Higgins, signed by Gov. Maura Healey in 2024. Beccia, of Lynnfield, is a financial-technology executive who loaned his own campaign roughly $3 million and ran the race's first television ad, built around the tagline "You Beccia." Belsito, of Topsfield, ran on a "People's Pledge" rejecting corporate PAC, super PAC and AIPAC money. Andres-Beck, 40, of Middleton, a software engineer at the health care technology firm Aledade, entered the race first, in mid-2025, originally intending to challenge Moulton directly over his comments on transgender athletes in youth sports; they ran in the primary as a Democratic Socialist once the seat opened.
The money behind the six Democrats was never close to even. Koh raised about $4.5 million, the most in the field, much of it in large checks from Boston and New York donors; about $271,000, or 6.5 percent, came from inside the district, according to the Marblehead Independent's review of Federal Election Commission filings. Beccia loaned his campaign roughly $3 million of his own money, with an estimated $7,300 — under 2 percent — from district donors. Nguyen raised about $711,000 with no candidate loans, roughly 85 percent of it — about $604,000 — from within Massachusetts. Belsito raised about $253,000 but drew the highest in-district donor share in the field, about 38 percent, or roughly $96,000. Andres-Beck raised about $322,000, mostly in small checks.
Outside groups added another layer in the race's final days. A pro-Nguyen super PAC, Pro-Choice Majority Action, reported spending about $500,000 on advertising in the campaign's last week; the outlet Sludge reported that part of the group's funding traces to a committee whose largest donor is the American Israel Public Affairs Committee's United Democracy Project, a detail in tension with Nguyen's own criticism of Israel's war in Gaza. A pro-Koh group, New Leadership PAC, reported spending more than $870,000 on Koh's behalf, including a $250,000 check from the sports-betting company DraftKings. Neither campaign controlled its outside spending, which by law must be raised and spent independently of the candidates.
Debate turns personal over money
During a June 17 debate at Salem State University hosted by the Frederick E. Berry Institute of Politics, the six Democrats broke from a night of near-total agreement when Belsito turned the conversation to money. Most people, she told the stage, do not have "access to the wealth that you do" — a comment aimed at Koh, whose immigrant father and young children he had invoked repeatedly through the evening. "For you to call out my family in this debate is something that is way past the line," Koh said. Belsito did not back down. "Your father's got a phenomenal job," she told him. Lancaster, then still in the race, pressed further: "Why do you talk about your kids on this stage?" Koh answered only that he was "proud to be a father."
Beccia, defending his self-funding to the same audience, said, "You can't buy an election," leaning on his "People Over Politics" pitch. Belsito called the underlying dynamic "class warfare" and urged voters to look at "what's going into people's campaign accounts." On policy, the six found more agreement than daylight: all six said they would abolish Immigration and Customs Enforcement, all backed universal health care and none would raise the Social Security retirement age. Asked how to pay for it, Andres-Beck had a one-line answer: "AI should pay for it."
Endorsements and exits reshape the race
The race's final two weeks brought a wave of endorsements toward Nguyen: U.S. Reps. Ayanna Pressley and Lori Trahan on Aug. 24; the Boston Globe's editorial board on Aug. 25; the Marblehead Independent's editorial board on Aug. 27; Attorney General Andrea Campbell; and Lancaster's withdrawal and endorsement on Aug. 29. Koh built a different coalition earlier in the year, drawing endorsements from former President Joe Biden, former Vice President Kamala Harris, former Transportation Secretary Pete Buttigieg, Walsh, Tierney and unions including the Massachusetts AFL-CIO and Teamsters Locals 25 and 3.
Two Democrats left the race before voting began. Rick Jakious, Moulton's former chief of staff, withdrew April 15, saying, "Running for Congress requires access to significant wealth," and endorsed no one. Lancaster, a Salem veterinarian, withdrew Aug. 29 — three days before the primary and after early and mail voting were already underway. "This is the election of a generation and, in the final days of this race, we must come together to ensure the Sixth District is represented by someone who will champion our progressive values," Lancaster said. Nguyen thanked her former rival. "I am grateful to Mariah for their support of our campaign," Nguyen said. "We share a commitment to building a stronger future for our communities and fighting for the issues that matter most to working families." Because Lancaster quit after voting had begun, their name remained on Tuesday's ballot; they drew 2,660 votes, about 3.7 percent.
Koh is stepping into a seat Massachusetts voters have not filled without an incumbent on the ballot since Michael Harrington retired in 1978. Every race since — including Nicholas Mavroules' 1992 loss after an indictment, Peter Torkildsen's defeat by Tierney in 1996 and Tierney's own 2014 primary loss to Moulton — featured a sitting officeholder. The 6th District runs across 39 cities and towns from the North Shore and Cape Ann through the Merrimack Valley, and Republican Micah Jones of Essex, an Army veteran and securities attorney, was unopposed in the GOP primary.
Turnout was already running ahead of the state's pace before polls opened Tuesday morning. As of 7 a.m., 555,525 of the state's 5,184,354 registered voters — about 10.7 percent — had cast a ballot through early and mail voting, according to state ballot statistics. By the time AP called the race, roughly 70,000 votes had been counted districtwide, with final tallies still being confirmed by city and town clerks overnight.
Koh advances to face Jones in the Nov. 3 general election and, in a district the Cook Political Report rates 11 points more Democratic than the nation, is heavily favored to win — potentially for a long tenure, given that Tierney and Moulton together held the seat for more than 30 years.
Koh gave no sign of coasting into that race. "We're still two months away — not from the finish line, but from the starting line," he said, telling supporters the campaign would "go anywhere, talk to anyone" across the district before November.
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