Gore was the underdog in 2000. I remember, I had +250 odds. The last polls had him, on average, behind around 3-4 points nationally. But a race without an incumbent is a different animal. Take a look at all incumbent races in recent history. All kept or booted the incumbent based on how he did, even as the challenger's credentials were questioned:
96: Clint was popular and won re-election. It was nothing against Dole, who was viewed positively.
92: Bush I lost because of the economy, stupid. There were huge questions about Clinton's readiness. But the voters rejected Bush.
84: Reagan was extremely popular, nuff said.
80: Carter was viewed as a failure. Again, big questions about whether the former actor was qualified and ready. But bottomline, Carter was rejected.
76: Ford was punished for pardoning Nixon. It wasn't that the people fell in love with Carter.
It's all about the incumbent. Bush's approval ratings just aren't high enough.