Dan Koh holds a commanding lead in the Polymarket odds for the September 1, 2026 Democratic primary in Massachusetts’ 6th Congressional District, an open seat created by incumbent Seth Moulton’s Senate bid. Traders appear to price in Koh’s early ballot qualification, substantial fundraising edge exceeding $3.5 million as of March, and name recognition from his Biden administration role, alongside high-profile endorsements that have consolidated support in a crowded field. Recent internal polling shows state Rep. Tram Nguyen gaining ground with 28 percent support and narrowing the gap, yet undecided voters remain elevated and Koh’s structural advantages continue to anchor the market consensus at 71 percent. Other contenders trail further behind amid the primary’s September timeline.