That tenuous balance of power may soon change. The 5-4 split that has defined the court in recent years could be altered with the replacement of a single justice. And since it seems likely that one or more justices will retire in the coming four years, the next president may have the rare opportunity to sharply tip the court’s scales to the right or left, perhaps for decades to come. Until recently, the Supremes have remained a sleeper issue in the presidential race. But as the sparring intensifies, the battle over the future of the court could emerge as one of the most hotly contested issues of the campaign.

As things are now, the court’s most conservative members–Chief Justice William Rehnquist, and Justices Antonin Scalia and Clarence Thomas–usually vote together on hot-button social and political issues like affirmative action, states’ rights and prayer in school, and are often joined by more centrist conservatives Sandra Day O’Connor and Anthony Kennedy. The more liberal justices–John Paul Stevens, David Souter and Clinton appointees Ruth Bader Ginsburg and Stephen Breyer–usually band together to dissent. Change that balance by just one vote, and the court could suddenly be much more liberal, or much more conservative.

Suppose, for instance, that Al Gore is elected. His appointee’s votes could reverse the trend away from race and gender preferences and cement a liberal majority on church-state issues such as prayer in school and tuition vouchers. He or she could also join the four liberals who have openly suggested that if they get a fifth vote, they will put a halt to Rehnquist’s mission to expand states’ rights. Big cases like the tobacco ruling, which pleased conservatives who want to rein in federal agencies’ regulatory powers, would probably come out the other way if one of the conservative justices were replaced by a Democratic president. Gore, pledging to remake the court if elected, sharply criticized the conservative justices for the tobacco ruling. “The Supreme Court is at stake,” he warned. “Many of our personal liberties are at stake.”

On the other hand, if Bush wins and has the opportunity to replace a retiring liberal, his (presumably conservative) appointee could help outlaw almost all governmental race and gender preferences. He could also loosen restrictions on church-state links, including aid to religious schools, and march further down Rehnquist’s states’-rights road. A change in the ideological balance could also swing the court to the right or left on campaign finance, gay rights, the “right to die,” privacy, crime, freedom of speech, property rights and more.

Interestingly, Roe v. Wade–still one of the court’s most divisive rulings–would likely survive. Six of the current justices support abortion rights, with O’Connor and Kennedy joining the four liberals. Abortion rights would be seriously threatened only in the unlikely event that two of these six retired, and both were replaced by justices bent on overturning Roe. Conversely, a Gore presidency could cement a more solid pro-Roe majority.

Of course, it is entirely possible that either Gore or Bush would let ideology take a back seat to other concerns when picking a court appointee. They may think it more important to nominate the first Hispanic justice, even if the nominee’s views don’t always match the president’s political agenda. No matter how carefully nominees are vetted for their beliefs, they don’t always vote as the president who appoints them might hope. Kennedy and O’Connor, both Reagan appointees, and Souter, a Bush pick, have all voted to uphold Roe v. Wade. At times, the court is simply unpredictable. “You shoot an arrow into a far-distant future when you appoint a justice,” the legal scholar Alexander Bickel once wrote, “and not the man himself can tell you what he will think about some of the problems that he will face.” Even so, whether that arrow is shot by a conservative or a liberal could make a very big difference.