Al Gore, the Popular Vote in 2000, and DUI charges.
Update I had a typo when I put in the swing in the margin. Voters who decided a week before the election favored Gore 48-45%, not 50-48%. That means that while Gore's support stayed the same, Bush's support dropped five percentage points. I think that is significant

I just heard for the ten thousandth time how formidable Al Gore was, and how he won the popular vote. Does anybody remember why he won it? Four days before the election, a partisan Democrat chose to reveal something he had been sitting on for months - Bush had a DUI. Bush went from being several points up in the polls to being several points down almost immediatly. Exit polls afterwards had that those voters who had decided in the week before the election favored Gore 48%-50%. Those who had decided beforehand favored Bush 50%-48%. Yes, other factors came into play, but let's not kid ourselves - Bush had been pulling away before the DUI story, and then it narrowed really quickly. So please, please, please don't tell me again how divided we are as a country. Yes, its close between democrats and republicans, yes there is a demographic shift in the Democrats favor countered by an overall mood swift to the right. But don't go back to the Florida vote in 2000 as support. If it hadn't been for that fluke of a story, none of that post-election mess would have happened.