After eight years as vice president and 16 years in Congress, Gore has rejected calls to run for office again.
"I don't think I'm very good at some of the things that the modern political system rewards and requires, and I've found other ways to make a difference and to serve the public interests," Gore told his hometown newspaper, the Nashville Tennessean, last year. "And I'm enjoying them."
"You've got to give Al Gore credit," said Shirley Anne Warshaw of the Centre for the Study of the Presidency. "He became a star by rising above politics with his passion for the environment. He's now international leader on an issue more and more people care about. There has been no more successful defeated presidential candidate."
To be sure, Gore still has critics, particularly ones who accuse him of overstating the threat of climate change, despite mounting scientific evidence.
Yet Gore has drawn support on both sides of the political aisle, including Democratic presidential candidate Barack Obama and John McCain, his Republican rival in the November election.
The two embraced Gore's challenge to commit to producing all U.S. electricity from renewable sources like solar and wind power within 10 years in order to get away from carbon-based fuels.
"If the vice president says it's do-able, I believe it's do-able," said McCain. Obama said, "It's a strategy that will create millions of new jobs ... and one that will leave our children a world that is cleaner and safer."
Sen. Jeff Sessions, an Alabama Republican denounced it. "Unless there is some monumental breakthrough, it is not possible," Session said. "It cannot be the basis of a sound energy policy by any responsible official in America, it seems to me. Maybe I am wrong, but I don't think so."
During the 2000 White House race, Gore was ridiculed as stiff and wooden. By contrast, Republican foe George W. Bush, then the Texas governor, came across as far more personable, although not as knowledgeable.
On Election Day, Gore won the popular vote. But Bush took the White House when a divided U.S. Supreme Court let stand his contested 537-vote margin of victory in Florida that allowed him to capture the decisive, state-based Electoral College.
At the 2004 Democratic Convention, Gore joked about it.
"You win some, you lose some and then there's that little- known third category," Gore said, drawing laughter, cheers and tears.
The Pew Research Centre released a poll in May that found 53 percent of Americans had a favourable opinion of Gore, slightly higher than Obama's 52 percent. Bush's approval rating is under 30 percent, battered by the unpopular Iraq war and the ailing U.S. economy.
