On the campaign trail, Republican John McCain and Democrat Barack Obama have outlined sharply different visions of the sort of justices they would pick to fill any vacancies that might occur:

• McCain has said repeatedly he would nominate justices in the mold of President Bush's appointees, John Roberts and Samuel Alito, "who have a proven record of strict interpretation of the Constitution." Among his most crowd-pleasing lines with GOP primary audiences was, "One of our greatest problems in America today is justices that legislate from the bench, activist judges."                                                                                                                                                                                                                                                                            McCain has avoided being more specific, but ever since Richard Nixon's campaigns, those two mantras have served as conservative code for selecting judges who would favor their views on curbing access to abortion, protecting business against lawsuits and regulation, allowing government-prescribed prayer in school or limiting judicial review of the powers of the White House or the Defense Department.                                                                                    

• Obama has held up as his model Earl Warren, the chief justice who steered a fractured court to the unanimous decision in 1954 that outlawed segregated schools. Warren also presided over an era of the court's expansion of protection of civil liberties and civil rights in numerous other areas.

  "Part of the role of the courts," Obama has said, "is going to protect people who may be vulnerable in the political process, the outsider, the minority, those who are vulnerable, those who don't have a lot of clout." He has also said that he "would not appoint somebody who doesn't believe in the right to privacy," the legal doctrine that is the foundation of Roe v. Wade.

The focus on Roe is particularly acute this year because the shrinking number of justices who have upheld abortion rights in recent cases are among the court's oldest members. Justice John Paul Stevens is 88; Justice Ruth Bader Ginsburg is 75 and has had major health problems. In addition, Justice David Souter is rumored to be weighing retirement. Abortion-rights opponents talk hopefully of being within one or at most two new justices of overturning Roe and allowing states to again outlaw abortion altogether.

After 35 years of polarized debate, abortion is not an area where there are a lot of undecided voters or potential side switchers. This page has long supported retaining Roe, with reasonable limitations, such as parental consent. More broadly, we've supported an ideologically diverse Supreme Court that reflects the nation's diversity of perspectives — one with a strong pragmatic center within the judicial mainstream and with a healthy respect for precedent. We supported the nomination of Chief Justice Roberts, and we were neutral on Alito.

A McCain victory, moreover, would not necessarily guarantee a reversal of Roe. For one thing, Democrats are expected to strengthen their majority in the Senate in November, making it harder to confirm any court nominee who might side with the justices who have been hostile to Roe. For another, given their views on respecting previous decisions, it is not certain that Roberts and Alito would vote to reverse Roe outright. Nor would an Obama win guarantee Roe would be protected. Many justices have departed from the views of the presidents who appointed them. Nevertheless, there's little doubt what either candidate would try to do.

Further, the next president could produce substantial change in established law in a number of areas beyond abortion. The court has divided 5-4 on cases involving school desegregation, gun control, the death penalty, the government's power to combat global warming and the rights of prisoners held at Guantanamo Bay, Cuba. Other one-vote decisions set new precedents in limiting punitive damages against miscreant corporations; shielding bankers, accountants and lawyers from disgruntled investors; and narrowing Congress' authority to regulate money in politics.

The retirement of Sandra Day O'Connor in 2006 and her replacement by Samuel Alito almost certainly shifted the balance of the court in disputes since then over abortion, religion and school desegregation, among other issues.

The types of justices McCain and Obama say they'd select offer a clear choice, and the issue merits more discussion during the remaining two presidential debates — particularly if the nominees can be nudged past sloganeering and into substance.

Given the scale of the problems the United States faces at home and abroad, the effect of the Nov. 4 election stands to be historic. Most immediately, the effects will be felt in the economy and in the wars in Iraq and Afghanistan. But the next president's choices for the U.S. Supreme Court has the potential to influence law and social behavior for a generation or more.