Democratic presidential candidate Bernie Sanders received the endorsement of a top union leader, a longtime president of the Communications Workers of America Larry Cohen, in a possible sign of progressive discontent with front-runner Hillary Clinton.

Cohen, who stepped down from his post in June after a ten-year presidency, signed on to be an unpaid volunteer for the Vermont senator's campaign. His reason was Clinton's ambivalence on Trade Promotion Authority.

Cohen told the Huffington Post that he "did everything I knew how to do to get Clinton to speak out on 'Fast Track,' " but she would not. Questioning her silence, the former union leader also decided to shift his allegiances to Sanders, the more progressive candidate who has repeatedly expressed support for union workers as well as opposition to Trans-Pacific Partnership.

"I frankly don't understand how you could be a major candidate for president of the United States — Hillary Clinton, or anybody else — and not have an opinion on that issue," Sanders said, criticizing Clinton in an interview with Bloomberg's Al Hunt on Charlie Rose's show.

Cohen plans to officially join the campaign by joining a rally in Council Bluffs, Iowa, later this week.