The Republicans were Catholics.
We will have to disagree.
I think they were largely White Anglo-Saxon Protestant. (WASP)
Catholics have always been democrats, the first election republicans won the Catholic vote was in 1972 when Nixon ran for reelection, it is disputed but possible that Eisenhower may have won their vote for his reelection in 1956.
Protestants have always voted republican, except in 1932, 1936, and in 1964.