The approval of a self-administered abortion pill by the US Food and Drug Administration last week threw the knife-edge presidential election campaign into a domain of American politics the candidates had hitherto managed to avoid: the moral battleground.

A campaign so far marked by a fight for centre-field voters over the nitty-gritty of Medicare and education is suddenly set to be blown open by emotional issues that arouse political passions in America: gay rights, race, state power, the division between church and state.

This may not be a headline-grabbing election like those fought by Bill Clinton or Ronald Reagan, but it is the closest and most politically polarised for decades. And nowhere is that polarisation more stark, and the battle so close and bitter, as over these 'social' issues, and for the terrain on which they are adjudicated: the Supreme Court, where the busiest term for many years begins tomorrow.

The US Supreme Court and its appellate subsidiaries may not even get a mention in Tuesday night's first presidential TV debate between the candidates. But the appointments the next US president makes to its bench will have a greater impact on the core of society than either candidate could hope to have, with judgments far outliving their terms of office.

'For the Supreme Court, this is the most significant election of my lifetime,' said C. Boyden Gray, White House counsel under George Bush senior. The present Supreme Court, headed by chief justice William Rehnquist, is the most narrowly divided for decades. The court had been a conservative bastion, but two appointments by Clinton turned it into a closely balanced scale which the slightest weight can tip decisively to the right or to the left - which is exactly what Bush and Gore intend to do, in opposite directions.

The court is up for grabs, with the next president likely to make two or three lifetime appointments. Its most liberal member, Justice John Paul Stevens, is 80 and likely to leave during the next presidential term. Justice Sandra Day O'Connor, a 'floating' moderate, is also likely to leave soon. Justice Rehnquist himself turns 76 today.

The court's decisions on major issues over the past two presidential terms have invariably been settled 5-4, occasionally 6-3, with Clinton's judges, Stephen Beyer and Ruth Bader Ginsburg, all but cancelling out the ultra-conservatives Antonin Scalia and Clarence Thomas who consistently take a hard right-wing line which Bush junior loves.

The court tomorrow sets out on a term with several crucial cases, concerning free speech, labour and disability law, federal civil rights, search-and-seizure laws, citizenship rights of children born to only one US parent and prisoners' rights. And now, inevitably, abortion.

Thursday's decision to license the abortion pill sent the anti-abortion lobby into a flurry of soul-searching and discussion of new tactics, with many pledging that legal action - likely to end up before the Supreme Court - will be among them. There is no issue over which Gore and Bush are more at odds - and no issue over which they are as anxious to ensure a place for their point of view on the Supreme Court bench.

It was the Supreme Court which, in the historic Roe v. Wade ruling of 1992, established every woman's right to terminate a pregnancy. And while the Food and Drug Administration has passed the abortion pill, three votes on the Supreme Court bench are ready to overturn 'Roe v. Wade'. Other conservative judges stand by Roe - but by only five votes to four.

One appointment could shift that balance, and moderates may be reluctant to remove the many barriers to abortion - and to getting the pill - that state authorities can still place on the process.

Apart from abortion, the most controversial area of recent Supreme Court rulings has been the death penalty, a matter of disagreement between Gore and Bush. There is a sway in public opinion (especially the legal profession) against the death penalty. The Rehnquist court has ruled extensively but narrowly on death penalty cases: it has allowed for the execution of the mentally retarded and of children.

'For the 3,682 men, women and children on death rows across the nation,' says Stephen Bright of the Southern Center for Human Rights, 'this election is literally a matter of life and death. The court could play a greater role in restricting the use of the death penalty, or could give the states free rein to carry out more and more executions'.

Underlying the relationship between the court and the issues of both abortion and the death penalty is the notion of 'states' rights' and federalism - the core of American constitutional politics. Justice Rehnquist has made the defence of 'states rights' his hallmark, and his Supreme Court has knocked back the expansion of federal law to a degree that even prevented the criminalisation of possession of guns in school.