There are two possible variants:
In this timeline John McCain is disqualified from the Presidential elections after nomination - his birth in Panama, to American parents is judged unsatisfactory as far as the qualifications for the President being a natural born citizen of the United States.
The ambiguities associated with Barack Obama's birth - including his failure to make the long form known and Hawaii's repeatedly changing birth certificate form - lead to his being forced to step down.
Post election crisis
In the alternative versions - either candidate is elected, but is forced to step down after a constitutional courts rules that they are barred from office.