The concert, billed as Deadheads for Obama, would be the first time the band performed together since they held a reunion tour in 2004. The band dropped "Grateful" from its name to honor the memory of its lead singer, Jerry Garcia, who died in 1995., and also has called itself The Other Ones.
The band, calling the last-minute show a one-time-only event, said they came together the night before the Super Tuesday primaries because the Obama campaign asked them to hold a get-out the vote rally.
"I think this is the first time for us," Lesh said, when asked what other political candidates the legendary band has so publicly supported.
Lesh's 18-year-old son is a volunteer on Obama's campaign. Obama taped a short introduction for the concert that the band planned to play before the show.
Former President Bill Clinton was also in San Francisco on Monday, stumping for his wife, Sen. Hillary Rodham Clinton.
The latest California Field Poll shows Obama and Clinton in a virtual dead heat in the Democratic race.
California is the largest prize in the Super Tuesday contest in which voters in 22 states will choose their candidate in the Republican and Democratic contests.
The Dead were the latest big names to join the celebrity bandwagon for Obama. On Sunday, Maria Shriver, wife of Gov. Arnold Schwarzenegger, joined her cousin, Caroline Kennedy, and Oprah Winfrey, in an Obama rally in Los Angeles. It was hosted by the candidate's wife, Michelle.
Schwarzenegger has endorsed Republican Sen. John McCain.