What to Watch in the Massachusetts Primary Election

Senator Edward Markey, who began his congressional career in 1976, faces a challenge from Representative Seth Moulton.

Massachusetts Democrats will decide the latest head-to-head matchup between the party's moderate and progressive wings. This time, top progressives are backing the incumbent.

NBC News chief data analyst Steve Kornacki breaks down real-time vote tallies from primary elections in Massachusetts, while giving viewers a behind-the-scenes look at election night coverage.