Conservatives think that the potential for making appointments to the Supreme Court will work to McCain's advantage. While I agree that on some issues the public is to the right to the Court's present position, the public is also to the left of where McCain's appointments could take it.
Overturning Roe v. Wade so that abortion is banned outright in some states is one real possibility. Recriminalizing homosexuality is another that's received no attention, and whatever the general public thinks about gay marriage, I don't think they want homosexuals arrested in their homes.
Lawrence v. Texas ended that practice five years ago on a 6-3 vote. Current Justices Alito and Roberts each replaced one vote from the majority and minority of that decision. Had they been on the Court in 2003, it would have been a 5-4 vote for gay rights, leaving Kennedy (whose age will be 75 in the year 2012), Stevens (will be 92), Souter (72), Ginsburg (79), and Breyer (73) in the majority. A single McCain appointment theoretically could flip a future decision and overrule Lawrence.
Two loose ends - first, would they return to and overrule this precedent? Justices don't like to do this, although they might also consider it okay to do so here, considering that Lawrence itself overruled Supreme Court precedent that was 'only' 17 years old. I've no doubt that on the present Court, Alito, Thomas, and Scalia would overrrule. As for Roberts I'm less sure about his willingness to overturn, but it's very possible. My guess is that one appointment by McCain replacing the majority justices makes overruling possible, three appointments makes it virtually certain, and two appointments is a coin-toss.
Second loose end - would they get the opportunity? In law school I learned that it's not clear what happens to laws that are overruled by courts - do they become nullified and no longer exist, or are they existing but are unenforceable? Apparently that hasn't been decided. The Texas law prohibiting homosexual conduct is still on the books, so Texas hasn't done a cleanup and removed the law. I could see a conservative Texas sheriff and conservative Texas DA trying to revive the law during a McCain Administration. A good factual situation for them would be a homosexual rape case where the evidence makes it hard to prove there was no consent - the DA could just bring a homosexual conduct case against the defendant but not the victim. After losing all the way through the Texas Supreme Court, the DA then appeals to the US Supreme Court and we see what happens.
It could be a strange country - gay marriage legal in some states, while gay sex is illegal in others. I don't think criminalizing homosexuality is a winner for the Republicans.