Bernie Sanders has picked up his biggest union endorsement yet. On Thursday, the 700,000-member Communications Workers of America union gave its official nod to the Vermont senator and Democratic presidential candidate.

“Our politics and economy have favored Wall Street, the wealthy and powerful for too long,” CWA President Chris Shelton said in a statement. “CWA members, like voters across America, are saying we can no longer afford business as usual. Bernie has called for a political revolution – and that is just what Americans need today.”

The communications workers union is one of the nation’s largest private sector unions. In backing the self-described democratic socialist, it joins two smaller national-level unions -- the National Nurses United and the American Postal Workers Union. The announcement breaks a streak of major labor unions endorsing Sanders’ chief primary opponent, former Secretary of State Hillary Clinton. The former first lady boasts endorsements from the two largest teachers unions, the largest public employees union, the service workers union and a number of building trades unions.

The CWA emerged as one of the fiercest opponents of the Trans-Pacific Partnership (TPP), a proposed free trade agreement linking the United States with 11 other Pacific Rim countries. All three Democratic primary candidates say they oppose the TPP, although Sanders opposed the agreement long before Clinton did.

Larry Cohen, the former president of the communications workers union is a leading member of the Labor for Bernie group, a network of rank-and-file labor activists who have put pressure on their respective unions to back Sanders.

In its announcement, the union pointed to an online vote by “tens of thousands” of its members over who to endorse.

A handful of major unions have yet to officially back a primary candidate -- including, among others, the International Brotherhood of Teamsters, the United Steelworkers and the United Auto Workers. Many observers expect the umbrella AFL-CIO federation to hold off on making a presidential endorsement until the competitive primary season comes to a close.