AGAINST FALSE COMPLACENCY. Brad Plumer has a good piece about how any Republican President will almost certainly try to replace John Paul Stevens with a doctrinaire reactionary in the mold of Sam Alito. To add to his point, I think it's easy to be lulled into a false sense of security by the fact that most landmark liberal precedents survived a significant number of Republican appointments (even leaving aside the fact that this is, as Brad notes, a misleading metric - -the Rehnquist Court's strategy was to largely drain precedents of substantial content rather than overturn them outright). First, you will often hear the idea that many Republican appointments have been "disappointments," with the implication that it's more likely than not that a new swing appointment will be surprisingly liberal (indeed, a few people -- despite a complete lack of evidence -- made this claim about Alito himself.) But this is misleading -- conservative justices selected for ideological reasons have, in fact, mostly worked out as expected. The "disappointments" were all selected for primarily demographic and/or political reasons -- Earl Warren was promised an appointment for delivering California to Eisenhower, William Brennan (a well-known liberal Democrat) was nominated because Ike was trying to attract liberal votes, Sandra Day O'Connor was selected because Reagan promised a female appointment, and Harry Blackmun and Anthony Kennedy were third choices who were confirmable partly because they were correctly perceived as more moderate than the previous nominees. (Souter we'll get to in a second.) So attempts to nominate a staunch conservative are likely to be successful, and Brad is correct that, if anything, a president whose moderation leads to a distrust of cultural conservatives will be more constrained to nominate an ideological conservative.
Secondly, on the subject of Roe, Jan Crawford Greenburg's new book Supreme Conflict reminds us that, while it's tempting to see it as inevitable in retrospect, there was always a great deal of luck involved in Roe's survival in the first place. Had Reagan appointed Robert Bork first and then Antonin Scalia, it is very likely he would have gotten both; Scalia, who was the first Italian-American appointee and didn't have a paper trial that included things like arguing that the 1964 Civil Rights Act was unconstitutional, would have been much harder to stop. And David Souter was nominated instead of Ken Starr largely because of a strange confluence of internal machinations within the Bush administration that could have worked out differently at slightly different times. If a Republican president is given another opportunity to put a very conservative median vote on the Court, don't bet on getting lucky again.