In 1976, Muskie was a credible candidate. Ford had improved somewhat since taking office, but his pardon of Nixon, though the right decision in the long run, still left bitterness in much of the country in 1976. As well, Ford had a tendency for making missteps.
Liberal Muskie is pretty well shut out of the south. Sanford is not much help in the south, but is able to deliver North Carolina, where he is popular. Muskie picks up strength in the northeast, midwest, and west, to win the election.
(Actually, I doubt that Muskie would have carried North Carolina even with Sanford on the ticket, but I put it for Muskie on the map in recognition of Sanford)
Muskie/Sanford 298
Ford/Dole 240
i also doubt sandford would have been able to swing nc.
he really wasnt all that popular in the state, not in 76 at least. he was seen as too liberal on civil rights. he also was unpopular due to the 2% food tax he instituted while governor. furthermore, while president of duke university he was perceived as overly-sympathetic to vietnam war protesters.
that being said, he was extremely popular in the black community.