The competition invites supporters of U.S. presidential hopeful Barack Obama to create 30-second spots demonstrating what inspires them about the senator's candidacy.
MoveOn said it will then buy time to run the winning ad on national television.
"Since creative and grassroots energy has helped power Barack Obama's campaign from the start, this contest is a great way to deliver that message of hope to voters in Pennsylvania and across the nation," Eli Pariser, executive director of MoveOn, said in a statement.
"MoveOn's 'Obama in 30 seconds' ad contest is a chance for everyone, from aspiring filmmakers to armchair pundits, to raise their voices to put Obama over the top and help make history," Affleck added.
"I think this contest is a powerful way for ordinary citizens to be involved in an extraordinary moment in our history," said Legend. "I'm thrilled to be a part of it."
The ads, which can be submitted at http://www.obamain30seconds.org/ starting March 27, will be judged by a celebrity panel that includes Affleck, Damon, Legend, Stone, Moby, Steve Buscemi, Eddie Vedder and others.