I guess if the Iranian Hostage Crisis was dealt with earlier, Carter could edge out a very narrow electoral vote victory against Reagan/Bush in '80, which likely sets up Mondale for the Democratic nomination in '84. Theoretically, if Pat Robertson were to campaign as a Reagan-esque Conservative voice in 1984, he could win the primaries from Bob Dole or Paul Laxalt and win the party's nomination. Who his running mate would be, I guess Strom Thurmond or Jesse Helms would be a good fit.
If this happens, naturally Mondale and whoever runs with him (preferably one with charisma, IMO) attacks him as a far-right fundamentalist and wins the election in a near landslide.