This event is difficult to game. If Ruth Bader Ginsburg or John Paul Stevens had made such an announcement, it would be a little easier. They are reliable votes on the left wing of the Court, but both are intellectual light weights. Obama could replace them with reliable judicial liberals of greater intellectual caliber, and that would advance his cause. But that isn't easy to do. The sad fact is that relative few judges in the history of the court have had more than mediocre legal minds.
Souter is the only serious legal mind on the Court's left wing. President Obama can easily find a replacement vote, and that is what counts most. But if he replaces Souter with another Bryer or Ginsburg, that will leave all the intellectual weight on the right wing. Scalia, Kennedy, Alito, and (despite the hostile Press) Thomas, are all judicial thinkers and writers well above the Court's typical membership. That sort of thing can have consequences.
For a good example of what left and right mean on the Court, consider jurisprudence on the Free Exercise Clause. The First Amendment starts off like this:
What does that bit about "free exercise" of religion protect? Everyone on both sides of the Court agrees that this prohibts any government in the United States, Federal, State, or local, from singling out a belief or practice because of the religion with which it is associated. Thus, in Church of Lukumi Babalu Aye v. Hialeah, the Court unanimously struck down a municipal act that was clearly designed to prevent adherents of the Santeria Relgion from operating a church in Hialeah.
But in Employment Division of Oregon v. Smith the Court upheld an Oregon law that prohibited the possession and use of the peyote, even when this was part of the relgious practice of the Native American Church. Antonin Scalia, writing for Court, argued that so long as a law was neutral and generally applicable it did not violate the Free Exercise Clause, even if happens to interfere with some religious practice. Under Oregon law, peyote is illegal for everyone, regardless of the motive for using it. That is a good example of the Court's "conservative" decisions.
In the Hialeah case, Justice Souter wrote a concurrence that took issue with the Smith rule. He argued, if I understand him, in favor of an early rule. If an act of government has a substantial negative impact on a religious practice, then government must demonstrate that the act serves a compelling government interest. A law forbidding human sacrifice might have a negative impact on a religious tradition that demand such a practice, but government has a compelling interest in protecting human life, so such an act would withstand judicial scrutiny. But an act forbidding the ceremonial use of peyote might not.
Scalia was right. Souter was wrong. Scalia thinks the Court should act like an honest umpire: enforce the same rules on all the players. The Smith rule is easy to read and apply, and it is fair to everyone. Souter thinks that judges should act as angels in the outfield, making sure the games come out right. But his rule requires an endless series of judgment calls. When does a policy have a "significant" negative impact on religion? When military jets are allowed to fly over a pacificist church? And what government interests are "compelling?" Well, it turns out that a religious practice is significant when the Court likes it. And a government interest fails to be compelling when the Court doesn't like it.
I think Scalia is right about the role of the Court. It should be an honest umpire. Souter thinks it should try to right every social wrong it perceives. That is what is at stake in the next Supreme Court appointment.