You'd have to really fine-tune an AH scenario to make it happen, but it's certainly possible. Engineer it so that both the Democrat and the Republican are significantly unpopular by the late stages of the campaign. I think everyone can imagine a venal scenario to derail Clinton in October. For Bush you'd have to use more imagination, but it's never out of the realm of possibility for politicians to make career-killing mistakes. Just time both nosedives for October, have Perot stay in it the whole time, have him run a more professional campaign (and yeah, that probably means a more polished running mate even though that's not totally important), and as a personal plea from 9-year-old me, have him NOT PREEMPT THE YOUNG INDIANA JONES CHRONICLES WEEK AFTER WEEK TO RUN HIS STUPID INFOMERCIALS -okay sorry, I'm back in control now.
Ahem. And that's how you do it.