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In a statement, Massachusetts Nurses Association/National Nurses United President Donna Kelly-Williams cited Lynch's work "supporting a number of issues important to frontline nurses in Massachusetts." But the union also focused on his experience as a member of a union and his work on union advocacy.

“As a former iron worker and president of the Iron Workers Union, Lynch’s record on issues important to labor and working families is beyond dispute, and includes his commitment to supporting a progressive agenda to create jobs with living wages, provide quality housing, education and a secure retirement for all," she said.

Lynch is banking on union support in his challenge to Rep. Edward MarkeyEd MarkeyWHIP LIST: How many Dems will back Sessions? Confirm Inga Bernstein for the District of Massachusetts FCC takes aim at AT&T, Verizon over 'zero-rating' services MORE (D-Mass.) for the Democratic nomination in the special election to replace Secretary of State John KerryJohn KerryPalestinian leader: Moving Israel embassy could jeopardize peace process UN leader willing to meet lawmakers amid push to cut funding Will Rubio vote for Tillerson? MORE (D). He enters the race an underdog against Markey, who has a larger campaign warchest and substantial support from the Democratic establishment.

Union support, Lynch believes, will help him get out the vote on Election Day.

By his own count, he's received the backing of at least 44 local unions. In recent weeks, the Massachusetts carpenters union and the Massachusetts Building Trades Council have endorsed Lynch.

But not all unions have warmed to him; a number have taken issue with his more conservative positions on topics like abortion — Lynch is against abortion rights — and the administration's healthcare law, which he voted against.

The Massachusetts Teachers Association and the National Education Association have both backed Markey.

The MNA/NNU endorsement is significant, as it indicates some unions are willing to look past Lynch's healthcare vote, and it could also boost his liberal bona fides — the MNA was the first labor union to endorse Sen. Elizabeth WarrenElizabeth WarrenDems push for outside witnesses at Mnuchin hearing Schumer: GOP 'filling the swamp' by targeting ethics chief Confirm Inga Bernstein for the District of Massachusetts MORE (D-Mass.).

—This post was updated to better reflect Lynch's union backing.