"In this year's special Senate primary, Democracy for America is proud to put its full support behind Ed MarkeyEdward (Ed) John MarkeyRegulators seek to remove barriers to electric grid storage Markey, Paul want to know if new rules are helping opioid treatment Oil spill tax on oil companies reinstated as part of budget deal 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 Kerry2020 Dem contenders travel to key primary states When it comes to Colombia, America is in a tough spot 36 people who could challenge Trump in 2020 MORE. The group raised more than $373,000 for Sen. Elizabeth WarrenElizabeth Ann WarrenTrump's SEC may negate investors' ability to fight securities fraud Schatz's ignorance of our Anglo-American legal heritage illustrates problem with government Dems ponder gender politics of 2020 nominee 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.