Bernie Sanders picked up a major union endorsement Thursday from the American Postal Workers Union, or APWU.

The announcement is huge for Sanders, as his rival Hillary Clinton is being supported by the first- and third-largest unions in the nation: the National Education Association (NEA) and the American Federation of State, County and Municipal Employees (AFSCME).

The APWU represents 200,000 U.S. Postal Service workers past and present from across all 50 states. Before the union's board announced its decision to back the Vermont senator, he had only picked up the endorsement of the 190,000 members of Nationals Nurses United.

"We should judge candidates not by their political party, not by what they say, not by what we think they stand for, but by what they do," union President Mark Dimondstein said in a statement. "Applying that criteria, Sen. Bernie Sanders stands above all others as a true champion of postal workers and other workers throughout the country."

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