Supreme Court's Direction Hinges on Who Wins '08 Race

To that end, advantage McCain.

The ages of the justices -- Souter is the youngest on the left, and Antonin Scalia is the oldest on the right at 72 -- favor the likelihood that the first opening would come from the liberal side. Stevens, although robust and in good health, is the second-oldest justice in the court's history. The fourth liberal is Justice Stephen G. Breyer.

Replacing one of the liberal justices with a consistent conservative such as Roberts and Alito -- the two McCain has said would serve as models for his picks -- could have far-reaching consequences on issues such as abortion, church-state separation, racial preferences and executive privilege.

But, as other presidents have found, justices take seriously their lifetime appointments and their legacies.

"We know from history that people generally do not leave the court when they're going to be replaced by someone they don't think is very much like them ideologically," McGinnis said.

Even if McCain has the chance to replace one of the liberals, he would face a formidable obstacle if Democrats control more than 55 seats in the Senate.

"It would be impossible for him to get somebody who's extremely conservative confirmed in . . . a Democratic Senate," said lawyer Miguel Estrada, whose nomination to a federal appellate court was blocked by Democrats.

McCain's best bet to appoint someone close to his "ideological ideal point," McGinnis said, would be to nominate a woman or a minority, who might be more difficult for Democrats to oppose. The last three appointments to the court have been white men, and there has never been a Hispanic justice.

Although there is no doubt the candidates would appoint very different people to the high court and lower federal judgeships, they also present a striking contrast in how they might approach the job.

Obama taught constitutional law at the University of Chicago, known for its analytic approach to the law, and would rely on his own thoughts on constitutional theories, said Cass R. Sunstein, a former colleague now at Harvard Law School and an informal Obama adviser.

"He knows these issues," Sunstein said. "I'd be very surprised if he wasn't extremely involved" in choosing nominees," Sunstein said, though he added that he has not had conversations with Obama about the qualities the Democrat would seek in a nominee.

Obama opposed Roberts and Alito and has mentioned Ginsburg, Souter and Breyer as models, although it was unclear whether he was looking only at the current court, rather than past justices, for examples.