Bernie Sanders is expected to gain the endorsement from Communications Workers of America Union, NBC News reported.
The group represents about 700,000 workers nationally, making it by far the largest union to back Sanders yet.
The union's endorsement will be announced at a news conference at 11 a.m. Thursday at the union's headquarters in Washington. News of the endorsement comes as Sanders has lost out on a string of major union endorsements to Democratic front-runner Hillary Clinton, whose campaign now claims the support of unions representing 12 million workers.
