If Reagan had run and lost in '76, the party would look back on the previous five races, sees two conservatives tanking but Nixon winning twice as a relative moderate and nearly winning in '60 too.
No love for tax cuts ("voodoo economics"), more detente, etc.
Huge Democratic majorities in both houses, this time gunning for an opposition president. Stagflation, more energy crisis, Iran hostages all on his watch, etc.
Probably two terms for him, coming after 12 years of GOP rule that saw Nixon resign, Ford and Reagan both flop. In 1988, GOP turns to GHWBush or maybe Bob Dole?
Evangelicals who came into politics to back born-again Carter in 1976 are probably alienated by Ted Kennedy and turn to the GOP, but not sure how that plays out with moderates in ascendancy there.
That could lead to a pivot to the Religious Right down the line, I guess, especially if Kennedy reshaped SCOTUS to the left as Reagan did to the right?
Instead of Bill Clinton in 1992, someone like Jack Kemp wins?
But win or lose, Reagan winning the nomination in 1976 seems like it'd have undercut movement conservatism considerably. He was lucky to miss his chance then and come out in 1980 instead.