Sitting Presidents who face a strong primary challenge from inside their own party, then withdraw, generally leave the eventual nominee to lose the general election.
Harry Truman and Adlai Stevenson - 1952
Lyndon B. Johnson and Hubert Humphrey - 1968
IIRC, it was Bobby Kennedy who took LBJ out...until LBJ had him killed.
Very good. We win either way.
But the point is to make sure Obama suffers an internal challenge.