A number of commentators are playing a little loose with the facts.
Reagan did not endorse Ford at the convention, but his short speech after he lost was very gentlemanly and gracious. Reagan did later endorse Ford, as can be seen in this ad by Reagan:
https://www.youtube.com/watch?v=e2q9LjMxPcY
That as it may be, it certainly helped Carter eke out a narrow victory that year.
We can only hope history doesn’t repeat itself.