Excuse me; why is everyone assuming that Reagan wins the general election? I agree that he likely gets the Republican nomination. However, lets not forget that Reagan, outside of Republican circles, was seen as a dangerous radical up until his debates with Carter in OTL. In this ATL, that reputation is still going strong, and we are talking about an election which was the Democrats to lose. Although Carter won the election narrowly, he did so after blowing a very large lead over Ford.
Even assuming that Carter wins the nomination in this situation (which is likely, but not foregone), we still have a situation where the nation was looking for a change from what was viewed as a corrupt Republican administration (Ford, by agreeing not to run, is going to play into this narrative. Here the nation has an honorable, albeit viewed as bumbling, President who was a care taker and agrees not to run, and then the Republican exstablishment gives them REAGAN!?!).
Although I don't think Reagan Goldwaters it; I could see a situation where the Democrats win handedly, especially if they run someone besides Carter (even Carter likely could beat Reagan in this situation, although almost as narrowly as he won in OTL).
Of course, this leads up to an interesting situation in 1980; is it George Bush or Jack Kemp who runs against Carter? Both of them would be much easier to beat than Reagan in OTL, and would have some interesting butterflies.