“Congressman Markey has been a strong and effective advocate for public education and the rights of working men and women,” said Massachusetts Teachers Association President Paul Toner in a release.

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“He knows how to get things done. He is intelligent, hard-working and principled. If Ed MarkeyEdward (Ed) John MarkeyNet neutrality supporters predict tough court battle over FCC's repeal plan Avalanche of Democratic senators say Franken should resign Driverless car bill hits Senate speed bump MORE is elected, we are confident he will fight as hard for students and public education in the U.S. Senate as he has done in the House.”

Lynch has launched a bid for the Democratic nomination for Secretary of State John KerryJohn Forbes KerryLobbying world Kerry: Trump not pursuing 'smart' or 'clever' plan on North Korea Tillerson will not send high-ranking delegation to India with Ivanka Trump: report MORE's (D) seat, but he enters the race an underdog.

The former union worker and advocate is relying on heavy, active union backing to help him get out the vote and shore up the deficit in funding he's likely to face against establishment-backed and deep-pocketed Markey.

By his own count, Lynch has secured the endorsements of over local 30 unions in Massachusetts. But his controversial vote against President Obama's health care law in 2010, as well as his more conservative positions on some social issues — like his position in opposition to abortion rights — remain contentious in union circles.

His failure to secure the endorsements of the MTA and the National Education Association indicate he does not have union support entirely locked up, which could become an issue for him as the race heats up and he is forced to grapple with his underdog status.