Dan Koh leads the Democratic primary for Massachusetts' 6th congressional district—an open seat after Seth Moulton's Senate challenge—due to his commanding fundraising edge exceeding $3.5 million and broad endorsements from unions and local officials. Recent internal polls show state Rep. Tram Nguyen consolidating support among likely primary voters and narrowing gaps in some surveys, though high undecided shares persist ahead of the September 1 contest. Other contenders like John Beccia and Mariah Lancaster trail in resources and visibility, sustaining trader consensus around Koh while leaving room for late momentum shifts driven by endorsements or turnout in this crowded field.