Dan Koh leads the MA-06 Democratic primary field as the consensus frontrunner due to his substantial fundraising edge—nearly $3.5 million raised—and established name recognition from prior congressional runs and service in the Biden administration as deputy director of the White House Office of Intergovernmental Affairs. Seth Moulton’s October 2025 decision to challenge Sen. Ed Markey for the U.S. Senate rather than seek re-election created an open seat, prompting a crowded primary on September 1, 2026. State Rep. Tram Nguyen follows as the next closest contender through her legislative record and local donor support, though she trails significantly in cash on hand. Remaining candidates, including self-funded entrants and lesser-known challengers, face structural barriers from limited visibility and resources ahead of the primary.