The political action committee MoveOn said members voted to endorse Joe Biden, the presumptive Democratic presidential nominee, for president.
Biden, 77, won 82.4 percent of votes cast by MoveOn members, the group announced Wednesday.
MoveOn endorsed Sen. Bernie Sanders (I-Vt.) in 2016. Sanders lost to former First Lady Hillary Clinton in the Democratic primary.
“MoveOn’s millions of members are ready to mobilize together in support of Joe Biden, working to turn out voters in key states and ensure that Donald Trump is a one-term president,” Rahna Epting, the group's political action executive director, said in a statement.
“Donald Trump is racist, corrupt, immoral, and incompetent. He must be voted out. Joe Biden is a leader who listens, who is running on the most progressive platform in Democratic Party history, and whose election would create an opportunity for the big, structural changes this country needs."
MoveOn was launched in 1998. Its initial focus was to convince lawmakers and voters to move on from then-President Bill Clinton's affair with an intern.
Clinton, a Democrat, was impeached by the House of Representatives but acquitted by the Senate.
Biden in a statement to NBC said he was grateful for the support of MoveOn.
"The stakes in this election couldn’t be higher and MoveOn members will be critical to mobilizing voters in communities across the country to go to the polls," he said.
The campaign director for President Donald Trump said MoveOn's endorsement of Biden "confirms his sprint leftward in an attempt to appease the radical extremists in his party."
"The left-wing activists who endorsed socialist Bernie Sanders in 2016 have officially taken the wheel of Biden’s campaign. In their endorsement, MoveOn praised Biden for running on the ‘most progressive platform’ ever, which includes tax increases, the job-killing regulations of the Green New Deal, free healthcare for illegal aliens, liberal judges, and abortion-on-demand," Tim Murtaugh said in a statement.
"It’s no wonder he can’t unify Democrats behind his candidacy. Over and over again Biden has proven that he is too weak to stand up to fanatic leftists who are now calling the shots and will expect him to continue to fall in line."
MoveOn said earlier this year it would spend $20 million to mobilize members, inspire turnout, and "protect the right to vote from voter suppression attempts."
The group said it's zeroing in on key battleground states, including Pennsylvania, Florida, Wisconsin, and Colorado.