There are many reasons to hope that Barack Obama will be elected on Tuesday as the next president of the United States. Some of those reasons concern legal issues. He supports the recognition of the legal right of terrorist suspects to receive a fair hearing of the charges against them and he would abandon torture as a method for trying to protect the nation’s security.

Meanwhile, John McCain wants to appoint to the Supreme Court judges who would interpret the Constitution “the way our founding fathers envisaged”. As Whoopi Goldberg, the actress, asked him on a television programme last month, does she need to worry about “a return to slavery”? In practice, McCain’s election would mean conservative appointments to the United States Supreme Court that will set back constitutional law for a generation.

There are currently four conservative justices on the Supreme Court. Justice Antonin Scalia (the oldest of the group at 72), Justice Clarence Thomas, and the two men appointed by President George W. Bush: Chief Justice John Roberts and Justice Samuel Alito. There are four liberal justices: Justice John Paul Stevens, 88, Justice Ruth Bader Ginsburg, 75, Justice David H. Souter and Justice Stephen Breyer. The ninth judge, Justice Anthony M. Kennedy, 72, fits into neither camp.

As the National Law Journal pointed out in August in its review of the 2007-08 term, the important decisions were made by a 5-4 vote: upholding the right of individuals to possess firearms, rejecting the Bush Administration’s defence of the legal basis for detention without trial in Guantanamo Bay and prohibiting the death penalty for crimes other than homicide. Justice Kennedy was in the majority in all three cases. Replacing even one liberal justice with another conservative, as McCain has promised to do, would be likely to lead to the overturning of Roe v Wade on the right of women to seek abortions, and would result in a recognition of greater executive power and weaker constitutional rights.

Justices serve on the Supreme Court for life. So vacancies depend on death or retirement. George Washington made 11 appointments, the most by any president. Jimmy Carter did not make any. Some presidents are unable to secure the appointment of their first-choice candidate as the Constitution states that the presidential power is subject to “the advice and consent of the Senate”. George W. Bush’s nominee in 2005, Harriet Miers, his White House counsel, withdrew when a Republican-controlled Senate cavilled at the prospect of approving a candidate who had no obvious qualification for the job — other than the President’s testimonial that Ms Miers was “plenty bright”.

Some presidents are disappointed by the performance of those whom they have successfully nominated. President Eisenhower did not intend Earl Warren to become the great liberal Chief Justice of the 1950s and 1960s, or William Brennan to be so influential a liberal member of the court. When asked if he had made any mistakes in the White House, Eisenhower replied: “Yes, two, and they are both sitting on the Supreme Court.” Of the current liberals, two were appointed by Republicans: Justice Stevens by President Ford in 1975 and Justice Souter by President George H. Bush in 1990.

The next president can expect to fill at least two vacancies on the Supreme Court. And the lawyers he appoints will determine the content of constitutional law in the United States for a period well beyond the term of office of their patron. Alexander M. Bickel, the legal scholar, said in 1969 that “you shoot an arrow into the far-distant future when you appoint a Justice and not the man himself can tell you what he will think about some of the problems that he will face”. But as presidents, and their advisers, become more professional in their assessment of the legal philosophy of potential appointees, it will be a rare event in the future for the performance of a Supreme Court justice to surprise anyone.

Who sits on the Supreme Court matters for more than legal reasons. In December 2000, the poorly reasoned and politically partisan 5-4 decision in Bush v Gore overturned the decision of the Florida court that votes in that state should be recounted, and so determined the result of the crucial Florida ballot and therefore the presidential election. But for the Supreme Court, George W. Bush would not have become the President, and the world would be a different place today. Happily for the rule of law, the election next Tuesday is unlikely to be decided by the votes of nine judges in Washington.

The author is a practising barrister at Blackstone Chambers in the Temple and a Fellow of All Souls College, Oxford

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