Dan Koh defeated Beth Andres-Beck, John Beccia, Tram Nguyen, and Jamie Zahlaway Belsito in the Democratic primary for Massachusetts' 6th Congressional District on Sept. 1. Incumbent Seth Moulton (D) ran for the U.S. Senate to represent Massachusetts.

Central to the election were the candidates' policies to address affordability, how they would oppose President Donald Trump's (R) administration, and whether their experience in or outside of politics and Washington, D.C., qualified them for Congress. Zahlaway Belsito and Koh had experience in Washington, D.C., and argued that their understanding of the federal system would benefit the district. Nguyen, a state legislator, said she was the only candidate with current legislative experience. According to The Boston Globe, Andres-Beck and Beccia argued that their non-political backgrounds would provide "fresh, outside perspectives to Capitol Hill."

In coverage of a June 2026 candidate debate, The Boston Globe's Anjali Huynh wrote, "Candidates offered similar, if not identical, messages in different fonts throughout the night, making it feel more like a collaborative policy discussion than a contentious debate." On affordability, Nguyen emphasized worker protections, Beccia middle-class tax relief, Koh housing and child care costs, Zahlaway Belsito reversing tariffs, and Andres-Beck education and energy prices.

• Andres-Beck is a senior staff software engineer. Andres-Beck said, "America could have good things: functional healthcare, honest government, safe schools, and a future we can take pride in building. I'm running for Congress because I believe we can have those things, not because we behave or wait our turn, but because we organize, show up, and believe in one another."
• Beccia is the co-founder and chair of FS Vector, a financial services strategic consulting firm. Beccia's campaign website said, "John is tired of politicians making unkept promises and offering the same old solutions that don’t work anymore. He is running for Congress to do what he did in business: create jobs, protect consumers, and make our economy work for working people."
• Koh is the host of The People’s Cabinet podcast and a former chief of staff at the U.S. Department of Labor and senior aide in former President Joe Biden's (D) administration. Koh's campaign website said, "Dan Koh is running for Congress because Donald Trump is trampling our constitution and American way of life, and we need leaders fighting back to defend our Democracy, protect our rights, reduce the cost of living, and stand up for families and communities like ours."
• Nguyen is an attorney and represented the 18th Essex District in the Massachusetts House of Representatives. Nguyen's campaign website said, "Now Tram is running for Congress to tackle the high cost of living, expand affordable housing, protect fundamental freedoms, and restore trust in our democracy. Tram knows that we don’t have to choose between making life better for people, and standing up to Trump. We can, and we must, do both."
• Zahlaway Belsito is the founder of the Maternal Mental Health Leadership Alliance. Zahlaway Belsito said, "While Trump wages war on our democracy, too many Democrats argue about whether we need to abandon transgender Americans or abortion rights. I'm a Democrat who won't apologize for human rights, universal healthcare, and a fair economy with good union jobs. We don't need to choose between these values, we need to lead on them."

On Aug. 29, Mariah Lancaster withdrew from the election, but her name remained on the ballot. Lancaster endorsed Nguyen. Before the election, The Cook Political Report with Amy Walter, Inside Elections with Nathan L. Gonzales, and Larry J. Sabato's Crystal Ball rated the district Solid/Safe Democratic.

This was one of 59 races for the U.S. House of Representatives in which an incumbent did not seek re-election. Across the country, 23 Democrats and 36 Republicans were not seeking re-election as of the Sept. 1 primary. In 2024, 45 incumbents — 24 Democrats and 21 Republicans — did not seek re-election.