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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 MarkeyEdward (Ed) John MarkeyDemocrats search for 51st net neutrality vote Overnight Regulation: Dems claim 50 votes in Senate to block net neutrality repeal | Consumer bureau takes first step to revising payday lending rule | Trump wants to loosen rules on bank loans | Pentagon, FDA to speed up military drug approvals Dems say they have 50 votes in Senate to overrule net neutrality repeal MORE (D-Mass.) for the Democratic nomination in the special election to replace Secretary of State John KerryJohn Forbes KerryFeehery: Oprah Dem presidential bid unlikely Dem hopefuls flock to Iowa Change in Iran will only come from its people — not the United States 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 Ann WarrenWarren: Trump is a 'racist bully' Poll: Oprah would outperform Warren, Harris against Trump in California Democrats continue to dismiss positive impacts of tax reform MORE (D-Mass.).

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