Republicans should hope that SCOTUS legalizes it soon, so the issue goes away.
I think that would be a disaster for GOP candidates. Right now they can cop out and say "Of course
I support traditional marriage, I also believe it is best to leave it up to states" If the Supreme Court strikes down state laws, then there will be a lot of pressure from the religious right - which still holds an inordinate amount of power in the GOP - to support an amendment that at the very least overturns the decision, or more likely, double down on a Marriage amendment.