That he is a natural born citizen
Biden is a natural born citizen. His candidacy isn't based on any lie - at least none which cover up Constitutional violations. If Obama is found to be ineligible then Biden will almost surely take his place.
Don’t think that really matters. Even though it is now tradition that a presidential nominee picks his VP nominee, that VP is still nominated via a vote of the party, however symbolic and rote it may be.
If any of this non-a-citizen stuff bears fruit...and I have long bene dismissive of that ever happening...Biden will become the president. Barring a divine, miraculous change of heart by Democratic-leaning electors, McCain isn’t shacking up in the White House.
What happens to ‘natural born citizen’ status when one becomes a citizen of a nation that does not allow dual citizenship?