Part of the problem is that after JFK and RFK were killed, LBJ's political downfall over Vietnam, Chappaquiddick, and Humphrey's cancer diagnosis the liberal political establishment in the US was gutted. By the 1970s, the Democrats just did not have any particularly charismatic leaders who could hold back the nation's growing conservatism. Had Ted Kennedy been nominated in 1980, he would likely have lost to Reagan. Although Kennedy would not be personally associated with Carter's failures, the fact of the matter is he drunkenly drove a car off a bridge and someone died as a result. That was a cloud over his head for the rest of his life, and in particular it raised questions about his ability to discharge the awesome powers of the Presidency. And Reagan could still blame the Democrats generally for the poor economy and Iran Hostage Crisis, which would encourage undecided voters to cast their ballots for him over any Democratic candidate.
For the Democrats to win in 1980, then either Ford needs to win in '76 or a Democrat needs to be elected that year who governs competently over the course of their first term. Carter's lack of success as President was primarily rooted in his inability, indeed his refusal, to cooperate with the Democratic supermajority in Congress. So, the ideal person to lead the country in the late 1970s would either be someone with extensive legislative experience or an executive who had more experience than being a one term Governor in a state where the Governor was little more than a figurehead. Virtually all of the major Democratic candidates in '76 were Members of Congress, and any one of them would have had better relations with Congress than Carter. Mo Udall came the closest to defeating Carter in the primaries, and he had a long legislative career before '76, so I imagine that he would be the most likely alternative to Carter. If he enacts a major economic stimulus, has a better response to inflation, avoids the hostage crisis (or at least finds a way to release the hostages before election day), he would have a fighting chance against Reagan.