Hypothetically... What would happen if Obama was ineligible to become president before he is sworn in?

I think McCain would be the next pres. It seems to me like the person who got the most votes from the field of eligible candidates would be the winner. That would be a tough one to pawn off on the people though. Maybe they'd decide to have a new election. Biden wasn't elected so I don't think he'd get it. Tough question.