Democratic presidential hopeful Sen. Bernie Sanders got another boost from the party’s left wing on Tuesday, picking up the endorsement of MoveOn.org Political Action, a nationwide grass-roots group that pledged to mobilize on Sanders's behalf.

The group said its backing followed a membership vote in which Sanders was the favorite of nearly 79 percent of the more than 340,000 people who cast votes. Hillary Clinton received about 15 percent, and former Maryland governor Martin O’Malley was the choice of about 1 percent.

“This is a massive vote in favor of Bernie Sanders, showing that grass-roots progressives across the country are excited and inspired by his message and track record of standing up to big money and corporate interests to reclaim our democracy for the American people,” said the group’s executive director, Ilya Sheyman.

Sheyman said MoveOn is prepared to mobilize thousands of volunteers to make phone calls and solicit money on behalf of the Vermont senator, adding that the first priority will be to turn out its 43,000 members in Iowa and 30,000 members in New Hampshire. Recent polls have showed tight races in both early nominating states.

Last month, Sanders also won the endorsement of Democracy For America, another nationwide progressive group, following a similarly lopsided vote of its membership.

In a statement Tuesday, DFA executive director Charles Chamberlain said the pair of endorsements make clear that the "Warren wing" of the Democratic party is uniting behind Sanders, a reference to Sen. Elizabeth Warren (D-Mass.), who was urged to run for president by many activists on the party's left.

Clinton, by contrast, has locked down endorsements from much of the Democratic Party’s establishment, including key elected officials across the country. She is also leading Sanders in backing among national labor unions.

MoveOn’s endorsement of Sanders is the second in a presidential primary in the organization’s 17-year history. In 2008, the group backed Barack Obama but not until shortly before the Super Tuesday nominating contests.

“I'm humbled by their support and welcome MoveOn's members to the political revolution,” Sanders said in a statement.