WASHINGTON - Democratic presidential candidate Bernie Sanders on Thursday received his biggest labor endorsement yet from the 700,000-member Communications Workers of America.

The Vermont senator, a self-described Democratic socialist who has been lagging Democratic front-runner Hillary Clinton in union endorsements despite his populist campaign platform, said he "would have won a lot more national union support" if other national unions followed the same process as the CWA.

"What we are seeing is a lot of grassroots support in union after union in this country, but that support has not necessarily trickled up to the leadership," he said.

At an event where Sanders appeared with members of the union's executive board, CWA president Chris Shelton said the decision to back Sanders was a reflection of strong support for him by the rank and file, expressed in a survey of the members.

Several thousand members of the union voted "decisively" over the last three months to endorse the Vermont senator, Shelton said. The CWA executive board followed that lead with a unanimous Thursday morning vote, Shelton said, because of Sanders' stances on the financial sector, debt-free college, health care and "retirement security," among other issues.

Shelton declined to comment on other unions' endorsements of Clinton, which include those of AFSCME, the Laborers, and the Government Employees' Union. Her support from labor groups now represents the majority of the unionized workers in the United States. Shelton did, however, repeatedly allude to the scrupulously democratic process that led to his union's decision in favor of Sanders.

"An endorsement coming from me or from the executive board alone would have been an empty endorsement," Shelton said.

Other CWA official were more blunt in their assessment of the race. Clinton "is talking a lot of fluff," said executive board member Vera Mikell.

Sanders, by contrast, "supports our issues, supports working issues, supports union issues," said Anetra Session, another board member who wore a red T-shirt and a CWA lanyard.