The last Republican candidate to not win the Iowa Caucus and still go on to become President was George H.W. Bush in 1988. H.W. would win the 1992 Iowa Caucus unopposed since he was the sitting President and running for reelection.
He actually did win the Iowa Caucus in 1980 by 2% over Ronald Reagan who would go on to become the President that year.
Who did Bush lose to in the Caucus in 1988? Bob Dole knows it was Bob Dole with 37% and Pat Robertson with 25% to Bush’s 19%.
His son W. won the Caucus both times in 2000 and 2004.