The decision in Roe v. Wade (no matter how much I favor it) was extremely dubious on constitutional grounds. It was based on a new right–right of privacy that is not explicit in the constitution and not obviously implicit either. So if McCain (or Bush for that matter, by means of an interim appointment as was discussed recently on TSD) were to appoint another conservative, then Roe v. Wade is likely toast. And that is the only way a president can bring that about, but it is all too likely. Notice thought that would only allow, not require, a state to ban abortion. I imagine most NE states would not and then any but a poor woman could easily get an abortion just for the price of a trip to one of those states.
As for same-sex marriage, AFAIK there has never been a supreme court decision that bears on the subject. Some see an analogy with state laws banning inter-racial marriage. I don’t. Let me say that I’m in favor of allowing same-sex marriage; it is just that I reject the analogy. My guess is that if it ever were the subject of a supreme court decision, I would guess they would uphold state laws banning it. On the other hand, the strict constructionists (I think there is only one–Clarence Thomas, whatever the others claim) would have to admit that, under the “full faith and credit” clause, same sex marriages performed in jurisdictions where they are legal would have to be recognized in all states. It is possible that with Clarence Thomas’s vote they would so rule. But don’t bet on it.
It is hard to see how any action by the president, short of appointing justices who agree with him (and getting them through the senate) could bear on either question.
I have always been ill-at-ease over Roe v. Wade. At the time, I said to my wife that it was a weak reed and that it would cause the women’s movement to stop pushing the point by getting states to repeal their anti-abortion laws. And so it is proving. If not now, some future court will fail to uphold it. On the other hand, the growing religiosity in the US may make it impossible to get the state goverments to allow abortion. Back to coathangers, I guess.