The federal judiciary will become markedly more conservative if McCain wins and markedly more liberal if Obama does. This shift will affect the outcomes of cases involving a host of ideologically charged issues, including abortion; gay rights; affirmative action; the death penalty; the rights of suspected terrorists; gun control; property rights; the environment; regulation; and big-dollar lawsuits against business.

To woo conservatives who have long mistrusted him, McCain has bashed "activist judges" who "legislate from the bench." He has cited Bush-appointed Chief Justice John Roberts and Justice Samuel Alito as his models of restraint.

Obama, who taught constitutional law at the University of Chicago, voted against the confirmations of Roberts and Alito, saying that they too often side with "the powerful against the powerless" and lack "empathy" for ordinary people.

The replacement of a retiring liberal justice by a conservative McCain appointee, or of a conservative by a liberal Obama appointee, could give the Supreme Court an ideologically solid majority for the first time in decades and gradually make a dramatic impact on the course of the law. That's because the current Court is so closely -- and deeply -- divided. It has four liberals, four conservatives, and one justice (Anthony Kennedy) who swings depending on the issue.