With the possibility — and what some commentators say is the probability — that between one and three Supreme Court justices will retire during the next four years, the next president could have an enormous and lasting impact on the nation’s highest Court and its legal and social policy.

But often neglected in the hype surrounding the future makeup of the Supreme Court and the hot-button issues that frequent its docket, is the considerable opportunity for the next president to shape the legal landscape through appointments to the 13 federal appellate courts. The Supreme Court hears arguments in only about 75 cases a year, while the geographically representative federal appeals courts provide the final decision in more than 60,000 cases annually. For most Americans therefore, it is the so-called lower federal appellate court, rather than the justices in their fabled marble temple in Washington, that will truly be the “court of last resort.” This year, the potential impact of the next president on the federal courts of appeals is especially noteworthy.

The U.S. Constitution states that the president has the power to nominate all federal judges — not just (as one might infer from listening to campaign rhetoric) Supreme Court justices. Moreover, upon Senate confirmation, these judges also serve a life tenure, meaning that vacancies among the 179 federal appeals court judgeships and the 578 federal trial court positions provide one of the greatest opportunities for a president to create an enduring legacy.

This assertion relies on a conclusion, generally agreed upon and documented byseveral studies, that judges from different party affiliations reach distinct legal conclusions on key legal issues. This is not to say that federal judges work from party talking points; most take seriously their independence, which is, after all, the reason behind their constitutionally mandated lifetime tenure. Nonetheless, the choice of a judge by a president is an inherently political undertaking, and different judicial philosophies of constitutional interpretation can lead to vastly different results in judicial decisions.

For example, according to a study of the period from 1970 to 1994 by George Washington University law professor Richard J. Pierce, Republican-appointed judges considering environmental cases voted to deny standing (access to the court) to plaintiffs challenging decisions of the Environmental Protection Agency 79.2 percent of the time. In contrast, during that same period Democratic-appointed judges voted to deny standing just 18.2 percent of the time in those challenges.

Doug Kendall, the head of the Constitutional Accountability Center and co-director of the Judging the Environment Project, which has examined the environmental record of Bush administration appointees to the federal bench, notes an additional significance.

“The Supreme Court rarely reviews these cases, making the composition of the lower courts just as important as the Supreme Court when it comes to the enforcement of our fundamental environmental laws.”

But the impact that judges of differing ideologies and partisan affiliations can have covers the spectrum of legal issues. At a recent Senate Judiciary hearing, for instance, Cyrus Mehri, an attorney specializing in employment discrimination, cited several cases in which judges have closed the door for individual plaintiffs, including a decision by the U.S. Court of Appeals for the 5th Circuit in which that court “reached out” to overturn a jury’s decision in favor of a plaintiff whom it found had been discriminated against.

In the case, Septimus v. University of Houston, the assistant general counsel of the University of Texas applied for the position of associate general counsel. After her supervisor belittled her and refused to consider her for the position, she filed a grievance, which in turn led the supervisor to draft a confidential memo reflecting plans to retaliate against her, which he later acted upon. The employee ultimately resigned and sued her former employer. A federal jury found that she was forced to leave because of the treatment she received and awarded her nearly $400,000 in damages. But a panel of the 5th Circuit Court of Appeals reversed the decision by what Mehri called “shifting the legal standard.”

Even with such a clearly defined link between party and judicial philosophy, of course, the potential impact of a president’s appointments also depends on the quantity, geographic distribution, and ideology of the judges he is replacing. For a president to be able to appoint a new judge, a current judge must either retire or take senior status, or Congress must create a new seat. But all judicial vacancies are not equal. If a judge decides to retire and he is from the party of the president, that president’s appointment of the replacement is less likely to significantly change the balance of that court.

Indeed, this is one reason why if John McCain were elected president he would have a much greater opportunity to move the Supreme Court further to the right than Barak Obama would have to move it back to the center — the justices who are rumored to be thinking of stepping down are more liberal in orientation. (Although, ironically, two of them, Justices John Paul Stevens and David Souter, were appointed by a Republican president.)

Obama would likely have a greater opportunity to shape the federal appeals bench if elected president, however, because of the current makeup of those courts. Of the 13 circuit courts, two, the 2nd and 3rd Circuits, are evenly divided between Republican and Democratic appointees, three, the 1st, 4th, and 11th have a Republican majority of one or two judges, and of the others, Democrats have a solid majority on just one, the 9th Circuit, while Republicans control the rest.

Russell Wheeler, a visiting fellow at the Brookings Institution and a former Deputy Director of the Federal Judicial Center, recently completed a study of the status of the 179 current federal appeals court judges, focusing on which judges would be likely to retire, as well as the probability that Congress would create 14 new appellate positions. He found that an Obama presidency could potentially raise the number of Democratic appointees from 44 percent to 58 percent and significantly increase the number of appeals courts with Democratic majorities.

“On the Supreme Court, the best Obama can do is hold the ground and keep the Court in its current state, whereas McCain could really firm up the conservative wing of the Court,” Wheeler explained. “On the appeals courts, however, Obama would not be limited in this way. He could bring a greater degree of balance.”

But Wheeler’s study also points out the significant reward from potential appointments to the appeals court bench that a McCain presidency would reap, with the number of Republican appointees rising to a virtually unprecedented 74 percent, as well as likely control of all 13 courts of appeals.

Ideological balance on these courts is reflected in two ways. First, judges are assigned randomly to the three-judge panels that hear most of the arguments. The makeup of a panel affects not only its decision but also its magnitude. Studies have shown that panels with judges appointed from different parties are less likely to reach sweeping ideological decisions as are panels on which the judges are of the same party. Secondly, a party’s control of the circuit can be significant on those occasional instances when a court grants en banc review, that is a review of a panel’s decision by the full appeals court.

Though much of the “science” of judicial appointments involves the very unscientific reading of tea leaves, there is little dispute that the next president will have a significant opportunity to shape the federal courts. Americans would do well to gain greater understanding of and pay more attention to this subject as they prepare to choose a new leader. As Justice Robert Jackson wrote, “It is not the function of the government to keep the citizen from falling into error; it is the function of the citizen to keep the government from falling into error.”