The closely matched probabilities across turnout brackets for the November 2026 Massachusetts Senate general election reflect uncertainty over baseline midterm participation in a safely Democratic state. Incumbent Ed Markey holds consistent primary leads over challenger Seth Moulton ahead of the September 1 Democratic primary, which could modestly increase voter engagement through heightened intra-party competition and associated get-out-the-vote activity. Historical midterm turnout rates near 49 percent of eligible voters, combined with limited general-election competitiveness, typical weather patterns, and varying mobilization by Democratic organizations, keep the range of plausible vote totals broad. Developments such as primary results, shifts in national political attention, or late registration drives could separate the brackets by altering enthusiasm among independents and lower-propensity voters.