What if Obama wins, and wins only by fraud?

What happens if, first, Obama were to win this election, and second, it is proven at some not-too-much-later date that he won by dint of election fraud (thank you, the exportation of Chicago-style politics of ACORN, et al)? Does the nation have to live through an impeachment proper, or does he simply get removed? Does the country have to go through another election to replace him, or does McCain get the office by default? How does our Constitution cover that? How do our courts deal with such a possibility? What would that do to the two-party system we have? How will our nation survive such an assault from within?

In light of all the cases now coming up in the pivotal states, where investigations have been launched, where Obama-allied ACORN offices have been raided, where evidence is already clear that there is fraud, I don't think this is an entirely unreasonable set of questions.