Long after George W Bush and his largely discredited administration are history, a legacy of their partisan zeal and ideological extremism will remain: a Supreme Court frequently controlled by conservative justices appointed by Republican presidents. This is the "long tail" of America government, analogous to the insurance risks that come back to plague insurers decades after policies were written.

At the moment conservatives have a reliable five-to-four majority on the court. As one of the liberal four is justice John Paul Stevens, who is 87, the possibility cannot be discounted that President Bush, before he leaves office, may have the opportunity to increase that majority by one.

The American constitution is based on the principles, first preached by English Whigs and Enlightenment French philosophes like Baron de Montesquieu in the 18th century, of separation and balance between the three "branches" of government: the executive, the legislative and the judiciary.

At the apex of the federal judiciary stands the Supreme Court. Its nine justices are nominated by the president, but must be approved by the legislature in the shape of the judiciary committee of the senate.

The court polarised

For many years this system has been close to crisis. The problem arises from the sharp ideological differences that arose between conservative judges drawn to "strict construction" of constitutional texts and often influenced by a conviction that liberal judges had erred because of their wish to use the law as an instrument of social engineering, especially in issues related to race and sex.

Godfrey Hodgson was director of the Reuters' Foundation Programme at Oxford University, and before that the Observer's correspondent in the United States and foreign editor of the Independent.

Among his books are The World Turned Right Side Up: a history of the conservative ascendancy in America (Houghton Mifflin, 1996); The Gentleman from New York: Senator Daniel Patrick Moynihan (Houghton Mifflin, 2000); and More Equal Than Others: America from Nixon to the new century (Princeton University Press, 2006).

Among Godfrey Hodgson's openDemocracy articles on American politics:

"Can America go modest?" (October 2001)

"After Katrina, a government adrift" (September 2005)

"Oil and American politics" (October 2005)

"The death of American politics" (October 2005)

"The next big issue: inequality in America" (13 September 2006)

"America against itself" (19 February 2007)

"Democracy in America: the money trap" (27 March 2007)

"Queen Elizabeth meets President George" (9 May 2007)

"The lost leader" (27 June 2007

The problem was perceived as acute as long ago as 1987, when liberal Democrats led by Senator Edward Kennedy succeeded in defeating President Reagan's nomination of the "originalist" legal scholar and judge, Robert Bork, who as President Nixon's solicitor-general in the Watergate affair had gone along with the dismissal of Nixon's critics. This led to the coinage of the term "to bork", meaning to attack a public figure in a vicious manner to prevent him or her being appointed to a post.

Since the ferocious fight over Democratic attempts to "bork" the conservative African-American judge, Clarence Thomas, in 1991, passions have risen so high over appointments to the Supreme Court and to lower levels of the federal judiciary that it has often been hard to get any appointments made.

President Bush has had the opportunity to appoint two new justices, including a relatively young new chief justice, John Roberts, who succeeded William Rehnquist after the latter's death in September 2005. Roberts is only 52, so he can expect to preside over the court for at least a quarter of a century.

The "Roberts court" finished its first term on 28 June 2007. Although it is widely conceded that chief justice Roberts is an exceptionally able judge who prefers to argue in legal technicalities rather than with ideological trumpet-blasts, it is also plain that in a quiet and clubbable manner he is nevertheless extremely conservative, and so is the court he has presided over.

Twenty-four of the court's decisions, more than in any recent term, were decided by five-to-four margins. And no fewer than nineteen of those, in the opinion of experts, were decided along ideological lines, reflecting a near stalemate between conservative and liberal judges that has led the minority liberals to find ways to make public their frustration. One liberal justice, Ruth Bader Ginsburg, for example, broke with custom by insisting on reading out a long dissent.

The five-to-four split has a peculiar characteristic. It reflects a four-to-four division with what is in effect the casting vote frequently going to the same justice in many important cases. The man in the middle is justice Anthony Kennedy, the man who replaced Robert Bork and another nominee who had to withdraw when it turned out that he had used marijuana in his youth.

Kennedy, therefore, came to the court as the nominee of a conservative president and with a conservative reputation. He has however disappointed conservative observers, one of whom (Matthew J Franck) describes him as "an impulsive sentimentalist" (whose) "prose style betrays (what passes for) his thinking, vibrating between the poles of maudlin hand-wringing and sanctimonious arrogance".

Like Sandra Day O'Connor, whose resignation from the court took effect in January 2006, Kennedy was a swing vote on the Rehnquist court. In the Roberts court, his influence will be greater. He has often held the court's decision in the palm of his hand. Kennedy has been in the majority in every single one of the court's twenty-four five-to-four decisions, and was in the minority in only two of the sixty-eight signed decisions.

The judges line up rather predictably like this. The chief justice and justice Clarence Thomas form a conservative block with reliable conservative ideologues in the shape of justices Antonin Scalia and Samuel Alito. The liberal four consist of justice Ruth Bader Ginsburg, Stephen Breyer, John Paul Stevens and David H Souter. Souter came to the court with the reputation of being a conservative, but as often happens (for example in the case of President John F Kennedy's friend, Byron White, who was appointed as a liberal but turned out to guarantee a reliable conservative vote) the outcome was different from what had been anticipated.

The five-to-four conservative majority held together on a wide range of important subjects, including anti-trust prosecutions, aspects of the death penalty and much else. The most surprising, perhaps, was the court's decision in June 2007 to hear a case bearing on the right of detainees at Guantànamo to have access to American law.

A reversal on race

The most contentious, and for liberals the most ominous, however, have been two cases which are widely interpreted as reversing, at least partially, a series of judgments heralded by the court's most famous decision of modern times: its historic 1954 ruling in Brown v Board of Education (of Topeka, Kansas), that legal segregation of the races was unconstitutional.

The cases, from Seattle and Louisville, Kentucky respectively, led to the court ruling against the use of racial criteria in allocating school places, even if the purpose was to prevent segregation. The chief justice's written opinion took the line that the only way to keep race out of such decisions was simply to ban the use of race. Liberals, however, fear that the decisions are another major step in the retreat from the Brown decision, which has been eroded by a series of decisions since the Regents of the University of California v Bakke case in 1977-78.

At that time, the pollster Mervyn Field, said that as a result of the court's decision "it has become much more acceptable to be less generous". Almost thirty years later, the court, thanks to the erratic but ultimately reliable stance of justice Anthony Kennedy, seems to be settling in for another long period in which generosity, in matters of racial and social equality, has nothing to do with it.