Al Gore has a lot of goodwill in the US and the rest of the world. He's widely perceived as not only deserving of the presidency in his own right, but as the person who was cheated of it. The minor point that the person who cheated him of the presidency in 2000 is today one of the most unpopular presidents in history should weigh even further in his favour.
That's it. I've nothing more to add. Obama-Gore. That's the dream ticket that will win the Democrats this November's election.
P.S. Apparently I'm not the only person who thinks so. Someone else has done a far better job of articulating why this may be the dream ticket.