Democratic presidential hopeful Sen. Bernie Sanders won an endorsement from the 200,000-member union representing postal workers on Thursday, his largest union endorsement yet. The American Postal Workers Union, which represents U.S. Postal Service employees and retirees, said Sanders' long-time dedication to unions' ideals won members over, reported The Associated Press.

"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," APWU President Mark Dimondstein said in an announcement backing the independent Vermont senator, according to The Wall Street Journal. "Applying that criteria, Sen. Bernie Sanders stands above all others as a true champion of postal workers and other workers throughout the country."

Dimondstein pointed to Sanders' efforts to keep open post offices and mail-sorting plants in rural communities, fight against postal privatization and opposition slower delivery standards, according to AP.

"Politics as usual has not worked. It's time for a political revolution," Dimondstein said, also highlighting Sanders' proposals for $15 minimum wage, free public college tuition and veterans' benefits.

"No other candidate has his record of fighting to defend and expand Social Security, promoting 'Medicare for all,' and opposing 'fast track trade authority' and rotten deals like the Trans Pacific Partnership (TPP)," Dimondstein added.

The endorsement is likely to give Sanders a boost ahead of the second Democratic debate in Iowa on Saturday. A CBS News/New York Times poll released Thursday showed front-runner Hillary Clinton leading Sanders 52 percent to 33 percent.

Sanders' views and priorities typically align more closely with those of unions, and he has remained one of unions' closest allies in Congress, however, Clinton remains far ahead in securing official support from organized labor, noted The Wall Street Journal.

The former secretary of state has already garnered endorsements from the American Federation of Teachers, the National Education Association and the American Federation of State, County and Municipal Employees, according to The Huffington Post.

Sanders' first national union endorsement came in August from National Nurses United, which represents about 185,000 members.

The biggest union - the AFL-CIO - is expected to wait until the primary is finished before endorsing a candidate.