"In this year's special Senate primary, Democracy for America is proud to put its full support behind Edward MarkeyEdward (Ed) John MarkeyDems lambaste Trump’s ‘outrageous’ EPA chemical safety pick Dems plan to make gun control an issue in Nevada Dem senator pitches ideas for gun control after shooting 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 KerryFor the sake of national security, Trump must honor the Iran deal Bernie Sanders’s 1960s worldview makes bad foreign policy DiCaprio: History will ‘vilify’ Trump for not fighting climate change MORE. The group raised more than $373,000 for Sen. Elizabeth WarrenElizabeth Ann WarrenOvernight Finance: Lawmakers grill Equifax chief over hack | Wells Fargo CEO defends bank's progress | Trump jokes Puerto Rico threw budget 'out of whack' | Mortgage tax fight tests industry clout Michelle Obama is exactly who the Democrats need to win big in 2020 Wells Fargo chief defends bank's progress in tense Senate hearing 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.