Since when wasn't it a core belief? Get out of the twilight zone. By the way, the GOP will not lose races because of a belief in creationism. That is the demorat party that you must be referring to.
Are you kidding me? You really think Barry Goldwater, the father of the modern conservative movement, was a creationist? I think it's pretty clear that no Republican President, from at least as far back as 1870 to 1980 was a creationist. Reagan wasn't himself a creationist, but he said nice things about creationists around election time. And that's that -- the entire history of creationism in the mainstream Republican Party. Note the absence of support in any Republican organization for the State of Tennessee in the Scopes Monkey Trial. Note also the electoral disasters that have befallen Republicans, in Kansas and Pennsylvania notably, when they decide that the Republican Party is really the Creationist Party.
Creationism is a religious philosophy and it is completely appropriate within the confines of your Church. It is not a political idea and should have nothing to do with a political party.