Don't forget 1980. They tried to dump Carter then because he wasn't liberal enough, and almost succeeded. Even if they thought Mondale wouldn't win, the Democratic wing of the Democratic party was likely hoping he'd be able to sow the seeds of a liberal resurgence in 1988. Still, I doubt they expected the disaster they got.
Here's 1984 with a 15% swing in the margin, he still loses badly:
A 19% swing is needed to eke out a narrow victory, carrying Michigan by less than a thousand votes:
(In a PV tie with uniform swing, Reagan still takes Ohio and Michigan for an apparent 306 EV mandate.)