MoveOn.org took a membershipwide vote before Super Tuesday, with two-thirds needed for an endorsement, and got 70% for Obama, according to spokesperson Adam Green.
The group held a similar ad contest for spots opposing George Bush in 2004. That resulted in the "Child's Pay" ad featuring children working in low-paying jobs to pay off the $1 trillion deficit.
MoveOn.org said the winning Obama spot will get a national TV ad buy and YouTube play. Plus, the winner gets $20,000 worth of video equipment.
Budding videographers and Obama supporters have until April 1 to submit their ad, which will be chosen by a "motley crew" of judges that includes Matt Damon and Ben Affleck, cyberspace superlawyer and academic Larry Lessig and the Rev. Jesse Jackson.