The future alignment of the Supreme Court - not Joe the Plumber or William Ayers - is one the most important issues in Tuesday's presidential election.

The justices most likely to retire next are two liberals: John Paul Stevens, 88, and Ruth Bader Ginsberg, 75. That means that if McCain follows through on a pledge to appoint conservative judges, a Supreme Court divided 5-4 on many hot-button issues will swing dramatically to the right. On the other hand, an Obama victory, even if accompanied by a Democratic landslide in the Senate, would mean that the court retains its current balance during Obama's first term.

Obama and McCain offer a stark contrast in the kind of justices they would appoint to the Supreme Court.

In a speech in May at Wake Forest University, McCain said: "I will look for accomplished men and women with a proven record of excellence in the law and a proven commitment to judicial restraint. I will look for people in the cast of John Roberts, Samuel Alito, and my friend the late William Rehnquist - jurists of the highest caliber, who know their own minds, and know the law, and know the difference."

Reacting to the Supreme Court's decision last year forbidding the consideration of race in determining which schools students could attend in Seattle and Louisville, Obama issued a statement saying, "Chief Justice Roberts' opinion reflects a disturbing view of the Constitution that equates voluntary integration with Jim Crow segregation - a view that is both legally and morally wrong. The policies that led to racially diverse schools in Seattle and Louisville are a far cry from the policies and racial subordination that led to blacks-only and whites-only schools in the pre-Brown [v. Board of Education] era. To equate the two is to turn a blind eye to our nation's history."

He added, "As president, I will appoint Supreme Court justices who understand the constitutional importance of Brown. Those justices will ultimately vindicate Brown's promise, as Justice [Stephen] Breyer and today's dissenters put it, of 'one law, one nation, one people, not simply as a matter of legal principle, but in terms of how we actually live.' "

The Supreme Court is divided into two factions. In the conservative camp are Roberts, Alito, Antonin Scalia and Clarence Thomas. On the liberal side are Stevens, Ginsburg, Breyer and David Souter. Depending on the issue, Anthony Kennedy votes with either side.

Of the nine sitting justices, seven - all except Ginsburg and Breyer - were appointed by Republican presidents. Eleven of the last 13 Supreme Court appointments have been made by Republicans.

If McCain wins the election and appoints yet another conservative to the bench - which will be more difficult if Democrats extend their advantage in the Senate, which confirms or rejects judicial nominees - it will have dire consequences for affirmative action and other policies.

In 2003, a 5-4 majority of the Supreme Court approved the use of race as a factor in the admission of qualified students to the University of Michigan Law School. In that case, Grutter v. Bollinger, Justice Sandra Day O'Connor supplied the key fifth vote.

O'Connor, who was the court's swing vote, retired and was replaced in 2006 by Samuel Alito, a solid conservative. The appointment of one more justice in the mode of Roberts and Alito will give conservatives a five-vote majority without Kennedy, the court's current swing vote.

Possibly more important than appointments to the Supreme Court are those to the lower courts. According to National Journal, only 0.1 percent of the 60,000 cases appealed each year are taken up by the Supreme Court. Republican-appointed judges hold 54 percent of the 674 full-time U.S. District Court judgeships, and 56 percent of the 179 seats on 13 Courts of Appeals circuits.

A McCain presidency would widen that Republican advantage, and an Obama administration would narrow it.

Meanwhile, a reshaped Supreme Court, in addition to possibly eliminating or severely restricting affirmative action, could leave its imprint on other major issues, such as abortion rights, gun control, government funding of religious schools, the death penalty, civil liberties, and consumer rights.

Tuesday's election is not only a choice between Obama and McCain; it's also a referendum on the future of the Supreme Court and the direction it will take the nation.