McCain doesn't run, Bush takes a larger beating on his DUI. Possibly more are uncovered. Keyes rises to the top of what's left to secure the nomination. OR
McCain and Bush are deadlocked at the convention, neither willing to compromise with the other. Keyes is offered as a compromise candidate, Bush backs this play. Keyes/Bush is the ticket.
Meanwhile, the stench of the Clinton scandals don't leave Gore. While his wooden and boring personality fails to excite. Double whammy, he loses.
Third option. McCain wins the nomination, gives Keyes the VP spot in an attempt to court more african-american voters. McCain has a heart attack the day before the election and dies. McCain still wins, Keyes sworn in, in his place come January.
It's all a stretch, but it's the best I've got that's anywhere near plausable.