Democratic presidential hopeful Sen. Bernie Sanders won the backing of the 700,000-member Communications Workers of America (CWA) union on Thursday, his biggest union endorsement yet.

After a three-month process driven by the union's rank-and-file members, rather than its leadership, the labor group's board voted Thursday to endorse the Vermont independent senator.

"They voted decisively for Bernie Sanders," CWA President Chris Shelton said at a conference in Washington, reported Reuters. "The executive board stayed out of this."

Shelton noted that the union has worked with Democratic front-runner Hillary Clinton in the past, but he said his members wanted a president who did not represent "politics as usual" and preferred Sanders' tough stance on big banks, according to The New York Times.

"CWA members have made a clear choice and a bold stand in endorsing Bernie Sanders for President," Shelton said. "Our politics and economy have favored Wall Street, the wealthy and powerful for too long. 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."

CWA - which has members concentrated in states like Ohio, California, Texas and New Jersey - is the third and biggest national union to endorse Sanders, after National Nurses United and the American Postal Workers Union.

Being that Sanders has said that he does not support super PACs, Shelton said that CWA will likely focus its energy into grassroots support rather than television ads.

The progressive group Democracy For America, which was founded by Howard Dean's brother Jim Dean, also endorsed Sanders on Thursday after nearly 90 percent of its members voiced support for him, a self-described democratic socialist, according to NPR.

Clinton has received the vast majority of labor support, winning the backing of 18 national unions or alliances, including the two largest teachers' unions, as well as the American Federation of State, City and Municipal Employees and SEIU. In all, more than 14.5 million union member have backed the former secretary of state, according to The Wall Street Journal.

Separately, Sanders' campaign announced Thursday that he has surpassed 2 million individual donations, more than any non-incumbent candidate in history and on track to beat a record set by President Barack Obama in his 2012 campaign, reported The Hill. Obama racked up 2.2 million donations by the end of his 2011 re-election campaign, and Sanders' campaign said that they expect to set a new record by the end of 2015.