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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 MarkeyDems push for more action on power grid cybersecurity Dem senator: Trump 'doesn't respect' the presidency Overnight Regulation: Labor groups fear rollback of Obama worker protection rule | Trump regs czar advances in Senate | New FCC enforcement chief MORE (D-Mass.) for the Democratic nomination in the special election to replace Secretary of State John KerryJohn KerryFrustrated Dems say Obama botched Russia response Budowsky: Dems madder than hell Tillerson: 'My view didn’t change' on Paris climate agreement 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 WarrenWarren cautions Dems against infighting What does the working class want? Better schedules. Live coverage: Senate GOP unveils its ObamaCare repeal bill MORE (D-Mass.).

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