U.S. Constitution, Amendment 20, Section 3:
Section 3. 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; and the Congress may by law provide for the case wherein neither a President elect nor a Vice President elect shall have qualified, declaring who shall then act as President, or the manner in which one who is to act shall be selected, and such person shall act accordingly until a President or Vice President shall have qualified.
As another poster pointed out Obama is NOT YET the President Elect per Amendment 20, Section 3. He does not ‘officially’ become the President Elect until January 6th, when the ‘electors’ present their nomination to Congress, upon which Congress affirms.
Right now all he has is a number of ‘electors’ getting ready to vote on December 15th.
The ‘media’ refers to him as the President-Elect, but in reality that’s false because the Electoral college has not yet met.
“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.