Hmm... Dick Cheney runs for President in 2008 and uses the Bush machine to discourage the rest of the Republican field from joining up? That's all I can think of for the GOP. For the Democrats... Obama's wife talks him out of a run by threatening divorce... and Bill Clinton is revealed to have had an affair with someone else, which causes so much embarrassment for Hillary that she decides to stay out.
Under this scenario, Dick Cheney would have the GOP in his pocket... and we'd probably see John Edwards as the Democratic nominee, seeing as how he was doing so well in the primaries before. Unless of course, John Kerry decides to see if his party would give him a second chance...
If Edwards becomes the nominee, Cheney would beat him in a close matchup after the scandal with Rielle Hunter hits the news. If it's Kerry, landslide Democratic win.