"In this year's special Senate primary, Democracy for America is proud to put its full support behind Ed MarkeyEd MarkeySenate Dem: Trump has to stop ‘reckless’ language on North Korea Trump sparks debate over war resolution for North Korea Foreign Relations Dem: North Korea is the modern-day Cuban missile crisis 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 KerryCongress needs to assert the war power against a dangerous president Sinclair and 'Big Media': The outrage that caused the outrage Tillerson sets a lost State Department on the right course MORE. The group raised more than $373,000 for Sen. Elizabeth WarrenElizabeth WarrenWarren: Education Dept lawyer may have violated conflict-of-interest laws Congress should think twice on the Israel Anti-Boycott Act Sanders plans to introduce single-payer bill in September 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.