Obama's approvals are better than Reagan's at the same point in 1988. If he sits at 52-53% come October then it's hard to imagine Clinton or any other Democrat losing.
Eisenhower had at least a 60% approval rating for most of 1960.
Eisenhower was still admired more for his generalship than for his Presidency. The 60% approval is consistent with his landslide wins in 1952 and 1956.
I have my theory on why Nixon lost in 1960: he was ugly. Not in behavior, but in appearance. Hillary Clinton doesn't look all that bad.
Add to that, few Vice-Presidents ever succeed their predecessor as President by through an election after the completion of the predecessor's term.