"Without the religious right there is no GOP party" - Not true. There may not be a GOP MAJORITY, but there would still be a party. Whether you want to hear it or not - public opinion just does not support the complete abolition of abortion at this time. If the GOP wants to be a minority party again, just outlaw abortion. We ought to work slowly at the margins, and not lose our cool. If we can send it back to the states (which is where it ought to be anyway IMHO), great. Seek to minimize the damage and make abortion less frequent and less tolerated.
You are correct but that does not change the fact that abortion is, in most cases, morally indefensible.
I am sure public opinion did not support revolution against England or the abolition of slavery but somehow we got those two things behind us.
It is a majority and it is a winning issue. Certainly if you frame it as an all or nothing question, it is a loser. But 2/3rds of the population favor more regulation of abortion. As long as rape, incest, and the life of the mother exceptions are included, the vast majority are against abortion. Pro-life is by a wide margin the single biggest one-issue voting block there is. Certainly, if you want to be dumb and call for a blanket ban on all abortions, you are gonna turn off most voters.