Having nearly won the Republican Primary four years earlier, when the 1980 Presidential Election rolled around, Reagan was a shoe-in to take on President Jimmy Carter. He won the Republican Primary that year easily, and then went on to beat Carter soundly in '80.
But let's say Reagan had, for whatever reason, decided not to run. I think Carter losing reelection is a foregone conclusion, so it's safe to say the Republican nominee would win that year, and become President. So who could replace Reagan in 1980?