Of all the things we talk about during a presidential campaign, the Supreme Court probably has the lowest discussion-to-importance ratio. Appointing justices to the Court is one of the most consequential privileges of the presidency, one that has become more important in the last couple of decades since the Court has become more politicized. But there isn't a great deal to say about it during the campaign, beyond, "If we lose the election, we'll lose the Court." The candidates aren't going to say much of anything about whom they'd appoint other than a bunch of disingenuous bromides ("I'll appoint justices who will interpret the law, not make law!"), and we don't actually know who's going to retire in the next few years, so in the campaign context there isn't much to be said .
But if there's anything that ought to make you afraid of a Mitt Romney presidency, it's this. First of all, if Romney wins he will be under enormous pressure to make sure that anyone he appoints will be not just conservative, but extremely conservative. Remember what happened when George W. Bush tried to appoint Harriet Miers: the right wing had a category 5 freak-out, not because they thought Miers was going to be a liberal, but because they couldn't be absolutely, positively sure that she wouldn't be a down-the-line Republican ideologue forever more. Unlike Romney, Bush had no particular need to prove to them that he was a real conservative, but the pressure was great enough that he eventually withdrew her nomination and nominated Samuel Alito, who was exactly what they wanted.
And that will be a shadow of the pressure exerted on a President Romney. So when he gets his chance to make an appointment, there is just no way he will do anything other than select someone pre-approved by the Republican base. And what kind of chance will he get? Well let's take a look at the ages of the current Court. I've arranged them from oldest to youngest:
Ruth Bader Ginsburg: 79
Antonin Scalia: 76
Anthony Kennedy: 75
Stephen Breyer: 73
Clarence Thomas: 63
Samuel Alito: 62
John Roberts: 57
Sonia Sotomayor: 57
Elena Kagan: 52
Of course, it isn't necessarily the case that the oldest justices will be the first to retire. A relatively young justice might become ill, or just get bored, and decide to go. And ideological considerations would probably affect that decision; if you were Ginsburg and Mitt Romney was president, you'd know that retiring would dramatically change the makeup of the Court, in a way you wouldn't like. But all else being equal, one would expect the older ones to be more likely to step down first. And health considerations might leave a justice with no choice.
So if Mitt Romney were president and one of the four liberal justices stepped down, it would be the end of 5-4 decisions. It would also be the end of all the "What will Anthony Kennedy do?" discussions, since Kennedy won't matter much anymore. There would be five highly partisan, ideologically ambitious justices who would have the majority on every question that came before them. If Kennedy retired during a Romney presidency, we'd be left with many 5-4 decisions, but they'd all be decided in the conservatives' favor, and the effect would be the same.
The Court hasn't had an ideological 180 since George H.W. Bush appointed Clarence Thomas to replace Thurgood Marshall in 1991 (though you might count Alito replacing Sandra Day O'Connor ). But there's a fair chance that we'll see one such shift in the next four years. If it happens when Romney is president, it could be the most consequential one in decades.