The best I can tell, many Republicans seem to be single issue voters.  He will lose groups of them if/when he crosses the only line they care about.  If he were to say that abortion is OK, then he would lose one group. If he were to say that gay marriage is OK, a different, though similar group would flee.  If he were to endorse some form of gun control, again a portion of his support would leave.  He is staying away from angering the single issue folks, so they stay with him.