Every four years, the first Monday in October brings fresh predictions that the outcome of the presidential election will tip the balance on the Supreme Court.

This time, that prediction almost certainly will come true.

Why is this year different? One reason is obvious: the next president can be expected to make at least one, and quite possibly more, appointments to the Court. Although that same statement was made four (and eight) years ago, the actuarial pressures for change on the Court are becoming irresistible. It has been 10 years since anyone left the Court, and the current group soon will have served together for a longer period than any nine justices in the nation's history. While members of the Court are noted for their longevity, Justice John Paul Stevens is now 84; Chief Justice William Rehnquist just turned 80; retirement rumors have long swirled around Justice Sandra Day O’Connor; and Justice Ruth Bader Ginsburg, while seemingly in fine fettle, has had well-publicized health problems.

But there is another, more surprising reason for anticipating a change in the Court's direction. Perhaps because of Bush v. Gore, or because Republican presidents appointed seven of the nine sitting justices, the current Court often is described as one that is dominated by conservatives. But that is not really so. In fact, there is a remarkable balance between the Court's conservative and progressive wings – which means that, depending on who is appointing the replacements, any change in the Court's composition will give it a substantial push in one direction or the other. Two departures from the same wing of the Court would cause a profound shift in the Court's center of gravity.

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