Draftgore.com, which describes itself as a group of grassroots Democrats, underwrote a full-page open letter to Al Gore in Wednesday's New York Times, imploring the former vice president to enter the presidential campaign.
The ad, which says 136,000 people have signed Draftgore's online petition, was published two days before this year's Nobel Peace Prize is expected to be announced. Gore has been nominated for the prize because of his campaign to bring attention to global warming.
Although Gore has said he has no intention of becoming a presidential candidate, several groups around the country are trying to persuade him to enter the race.
"America and the Earth need a hero right now — someone who will transcend politics as usual and bring real hope to our country and to the world," Draftgore's letter said. "Please rise to this challenge, or you and millions of us will live forever wondering what might have been."
As the Democratic nominee in 2000, Gore won the general election popular vote but lost the electoral vote to George W. Bush after a legal challenge to the Florida result that was decided by the U.S. Supreme Court.
Since then, the former Tennessee senator has worked against global warming and served on corporate boards, including Google and Apple Inc.
A spokeswoman for Gore and officials for Draftgore did not immediately respond to requests for comment. The Draftgore.com Web site claims to be an independent group not in contact with the former vice president.