Markey will be sworn into the Senate on Tuesday morning. His resignation from office, to fill the seat of Secretary of State John KerryJohn Forbes KerryBringing the American election experience to Democratic Republic of the Congo Some Dems sizzle, others see their stock fall on road to 2020 The Hill's 12:30 Report MORE (D), allowed authorities to schedule the race to replace him in the House.

Five Democrats have entered the primary so far. State Sen. Katherine Clark and Middlesex County Sheriff Peter Koutoujian are considered the frontrunners in the race, with Clark expected to receive the endorsement of EMILY's List, while Koutoujian led the field in second-quarter fundraising.

State Rep. Carl Sciortino and State Sens. Karen Spilka and Will Brownsberger are also vying for the nomination.