"In this year's special Senate primary, Democracy for America is proud to put its full support behind Ed MarkeyEdward (Ed) John MarkeyLawmakers prep for coming wave of self-driving cars Trump taps Hill veteran for White House environment job Dems unveil push to secure state voting systems MORE — the only tried and true progressive in the race. Democracy for America members know they can trust Markey, unlike his primary opponent, to be a reliable Warren ally in the Senate and continue his long record of standing up to conservative Democrats on health care reform and climate change," the group's political director Michael Langenmayr said in a statement.

DFA's endorsement adds to the growing list of progressive groups backing Markey's bid for the seat vacated by Secretary of State John KerryJohn Forbes KerryGOP Senate report says Obama officials gave Iran access to US financial system Democrats conflicted over how hard to hit Trump on Iran In Iraq, the US State Department has a religious blindspot MORE. The group raised more than $373,000 for Sen. Elizabeth WarrenElizabeth Ann WarrenTrump to nominate budget official as next consumer bureau chief Sessions floats federal law that would protect states that decriminalize marijuana Bank regulator faces backlash over comments on racism MORE (D-Mass.) during her 2012 campaign and plans to send out its first fundraising email for Markey on Monday.

The PAC will also canvass for the candidate, adding to efforts announced on Monday by the League of Conservation Voters to launch a field campaign for Markey.

The dean of the Massachusetts delegation faces Rep. Stephen Lynch in the Democratic primary, and starts the race ahead in the polls and in fundraising. Lynch faces an uphill battle, too, because of his more conservative positions on a number of issues, including opposition to abortion rights and his vote against healthcare reform.

Three GOP contenders are vying for their party's nomination, but the seat is likely to stay in Democratic hands.