The last time the Democrats made a last-minute change to their ticket was 1972 - it resulted in one of the largest presidential landslides in history (Nixon over McGovern). It wasn’t that Nixon was all that popular but that the Democrats were so dysfunctional that year.
I’d say that Nixon really was all that popular, but also that McGovern was such a lame candidate that Nixon’s popularity became squared and cubed. Why was Nixon popular? Because he was very active in counteracting a stalled economy. His price controls were the very opposite of conservatism, and didn’t do a damned thing for the economy. But in those days, people believed in federal interventionism.