With Rep. Seth Moulton's October 2025 shift to the U.S. Senate primary challenging Sen. Ed Markey—where polls show him trailing—Massachusetts' 6th Congressional District Democratic primary remains wide open ahead of the September 1 contest. Dan Koh leads trader consensus at 74% implied probability, propelled by his record $2 million early fundraising, high-profile endorsements from Pete Buttigieg and former Rep. John Tierney, and organizational momentum as the first candidate to secure ballot access on March 26 via Lynn caucus signatures. Recent candidate forums, like the March 2 Bedford-Billerica event highlighting health care and immigration divides, underscore a fragmented field of nine Democrats, with Moulton lingering at 12% on speculation of a potential House return, while others like Kevin Larivee and Jamie Zahlaway Belsito split remainder on local backing. No public polls yet; upcoming endorsements and FEC reports could shift dynamics.