Since primaries rather than smoke-filled rooms and caucuses became the predominant way for GOP to select its nominee (c. 1968), they've never nominated someone who was not: President, VP, son of a President, or runner-up in a previous GOP presidential race. Mitt Romney was runner-up in 2008.
See http://en.wikipedia.org/wiki/Republican_Party_presidential_primaries,_2008 et al.
