Anti-Catholicism as well as RN’s “experience” factor should explain much of 1960.
Imo Carter ran a poor campaign in 1976, with his victory map looking much like a regional one with wins in much of South and Northeast and not much else. Even in the South, Carter offended many of his fellow white evangelicals by agreeing to interview with Playboy and admitting to “lusting” after other women during his marriage
I recall reading somewhere that Carter led Ford in the polls by around 30 points in June 1976, around the time of the Democratic National Convention that year. And Carter's lead collapsed completely during the intervening months, and he only won by 2 points. Carter's collapse means that the Johnson landslide of 1964 will be the last sweeping Democratic landslide (in terms of counties carried) that we will ever see. If a Democrat wins a landslide victory again, it will probably resemble Obama's maps more than those of the past.
To be fair parties did have huge convention bounces back in the day so the polling lead was quite inflated(Remember Reagan had a 16 point lead after the RNC, Dukakis had 17 point lead in 88 and then after other party convention Carter had a 1 point lead and Dukakis's lead was dropped to 7 points)
https://en.wikipedia.org/wiki/Historical_polling_for_United_States_presidential_electionsReason for that is simple back in those days Conventions were the first time the vast majority of people were hearing about non incumbent candidates, and hearing about the election for the first time itself so having 4 consecutive days but nothing but positive press for each candidate is why there were such huge bumps.
Remember back in those days cable news didnt exist or had minor influence while it was the network news that had most of the influence which was just half and hour a night for the most part.