“If, at the time fixed for the beginning of the term of the President, the President elect shall have died, the Vice President elect shall become President. If a President shall not have been chosen before the time fixed for the beginning of his term, or if the President elect shall have failed to qualify, then the Vice President elect shall act as President until a President shall have qualified” ...
This says that the Vice President Elect will act as President until a new President qualifies.
Going back to the preceding election, if Obama was disqualified, then his electors are null and void. Therefore, the next qualified member in the presidential pool is McCain. He has 538 electors pledged to him. They would probably be seated and would presumably vote for him.
And they will not cross party lines for a pick.