Massachusetts election officials have scheduled a Dec. 10 special election to fill the soon-to-be-vacant 5th District.

The primary — which will likely pick the next congressman in this strong Democratic district — is set for Oct. 15.

Longtime Rep. Edward J. Markey, D-Mass., will be sworn into the Senate on Tuesday, succeeding Secretary of State John Kerry after the congressman’s  special election victory last month. But Markey’s election creates another special election for his House district.

Thus far, five Democratic candidates have announced they are running for the rare open seat:

• State Sen. Katherine Clark, who announced her candidacy even before Markey officially entered the Senate special election. Clark, who is poised to receive the support of EMILY’s List, is considered a front-runner in the race.
• Middlesex County Sheriff Peter Koutoujian. Koutoujian, who served in the state House prior to becoming sheriff, is also considered a top-tier candidate in the five-person contest.
• State Rep. Carl Sciortino, an openly gay candidate who has billed himself as the most progressive candidate in the race.
• State Sen. Karen Spilka.
• State Sen. Will Brownsberger.

The 5th District is the state’s most reliable Democratic seat; President Barack Obama won the district in 2012 with 65 percent.

The 5th District special election is rated a Safe Democrat race by Rothenberg Political Report/Roll Call.