If he were nominated and then shown not to be a citizen prior to the election, two courses of action seem open. First, an emergency party convention could be held and HRC would get the nomination. In the second course, they could continue supporting him at whatever the cost reasoning that they had gone too far to drop his nomination. The first case would run into all sorts of legal problems and contravene all sorts of party rules. The second case could ensure their defeat if McCain decides to use his opponent's lack of qualifications for the job as a campaign issue.
If Obama is actually elected and then shown not to be a citizen prior to the swearing in ceremony, the Chief Justice would presumably decline to swear him into office on the basis of his not being a citizen.
This brings up the final scenario. Obama is elected, shown not to be eligible for office and 'accidents' occur leaving only Justice Ginsburg or Souter alive. Then Ginsburg or Souter would swear him in in a New York minute. After all, that 'natural born citizen' stuff was written by dead white men in a different time, we live in a brave new multicultural world, and the Constitution, being a living document, needs to change with the times.
Sounds like the precedent for the mother of all amnesties.
Obama would be defeated as surely as Torricelli and Carnahan, you mean?