Obviously, when I think about the possibility of a McCain presidency, one of the first things that comes to mind is that Justice Stevens is no spring chicken, and his presence on the court is an important bulwark against a lot of conservative judicial activism, among which might include the overturning of Roe v. Wade. That said, Matt has a good post up about whether or not McCain could, in fact, ban the practice of abortion. Basic conclusion is that it's not very likely, and to that end, Matt cites Brendan Nyhan:
The president can’t make abortion illegal. If John McCain appointed new conservative Supreme Court justices (who must be confirmed by the Democratic Senate), it is possible that the Court could decide to overturn Roe v. Wade. In that case, the issue would be returned to the states, who would each create their own abortion policies through the legislative process. The odds of McCain successfully passing a constitutional amendment to create a national ban on abortion are zero — there is simply no way he “will make abortion illegal.”I remain convinced that proponents of reproductive rights should, in any event, seek to enshrine and maintain those rights through the legislative process anyway, wherever it could feasibly be done. There's still this general idea that Roe magically makes these rights self-evident and iron-clad, but in many states, as a practical matter, abortions are so difficult to obtain that a ban is almost superfluous at this point.