"I am incredibly proud that there is finally a candidate speaking to the values I've held my whole life," said one worker.
Clinton and Sanders largely agree on issues ranging from labor law reform to rebuilding infrastructure to changing education law.
The endorsement came after Clinton - and only Clinton - addressed the NEA's board for an hour and a half.
Carpenters president Douglas McCarron introduced Clinton as "the next president of the United States."
The Letter Carriers endorsement is important, as the union has members in every state in the U.S., plus Puerto Rico, Guam and the U.S. Virgin Islands.
Trumka was caustic about Trump's constant anti-immigrant and anti-Latino stands. He also criticized two other GOP hopefuls.
Machinist leaders unanimously endorsed former Secretary of State Hillary Clinton for the Democratic presidential nomination, making IAM the second union to do so.
Sanders knows his campaign is not about him; it's about all of us, and it will take all of us to change the course of history. It's time to start now.
Add New Jersey Gov. Chris Christie to the lengthening list of GOP presidential hopefuls who are open and vocal union-bashers.
Democratic presidential contender Hillary Clinton supports comprehensive labor law reform, and she says she really means it.