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With exception of the presence of a "states rights" 3rd party nominee, election is close to results of 1960. Romney makes gains over Nixon in Upper Midwest, while LBJ wins CA and Appalachia.
No way the election is that narrow. On Election Day 1964, LBJ had about a 65% approval rating. That translates to an LBJ landslide, at least in terms of the EV.
Okay, a good approval rating overall, but where is it located? I may have given Romey a couple too many states, but still there's the Southern problem after the Civil Rights Act of 64.
LBJ probably had enormous approvals anywhere in the U.S. outside the Deep South in 1964. Thus, I think that he would win at least in a Clintonesque landslide in a worst-case scenario.
Approvals don't always translate directly into victory. They could feel the challenger is better, even if they like the incumbent well enough.