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The liberal political group MoveOn.org threw its endorsement to Senator Bernie Sanders in the Democratic presidential contest, backing him after its officials spent months in search of an alternative to Hillary Clinton and invested $1 million in ads to draft Senator Elizabeth Warren for the race.

The group’s members voted in large numbers — 78 percent of its membership, according to a press release — to support Mr. Sanders, the Vermont senator. Of the 340,665 votes cast, Mrs. Clinton received 15 percent, and former Gov. Martin O’Malley of Maryland received 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,” Ilya Sheyman, MoveOn’s political action executive director, said in a statement. “MoveOn members are feeling the Bern. We will mobilize aggressively to add our collective people power to the growing movement behind the Sanders campaign, starting with a focus on voter turnout in Iowa and New Hampshire.”

The endorsement is not a surprise, given the time that the group has spent trying to find someone who could be a strong challenger to Mrs. Clinton from the left. But where the endorsement could be helpful is in motivating volunteers and supporters to make calls and rally voters on Mr. Sanders’s behalf.