Justice Sandra Day O'Connor, the first woman appointed to the US Supreme Court and a swing vote on abortion and other key issues, announced her retirement yesterday, setting off what could be the most bruising political showdown in years over her successor.
President George Bush, presented with his first opportunity to determine the court's ideological balance, is expected to nominate her replacement when he returns from the G8 summit in Scotland next week.
The battle to shape the court's future is expected to be fought with all the intensity of a presidential election, with both left and right using television and the internet to make their case.
"I will choose a nominee in a timely manner so the hearing and the vote can be completed before the new Supreme Court term begins," Mr Bush said.
"The nation deserves and I will select a Supreme Court justice that Americans can be proud of.
"The nation also deserves a dignified process of confirmation in the United States Senate, characterised by fair treatment, a fair hearing and a fair vote."
Justice O'Connor (75) has often cast the deciding vote on the nine-member court, and upheld the right to abortion in a 1992 ruling. Although appointed by Republican president Ronald Reagan, she has been described as a pragmatic and moderate voice on the Supreme Court.
This means, observers said, that if Mr Bush appoints a conservative, the court's balance will be changed and it will become more conservative.
Justice O'Connor is the first Supreme Court judge to retire since 1994 when president Bill Clinton appointed Justice Stephen Breyer, a liberal judge.
Her announcement took Washington by surprise as lawmakers and political commentators had anticipated that it would be Chief Justice William Rehnquist, who is 80 and suffering from throat cancer, who would step down this week at the end of the current court term.
Justice O'Connor voted with the 5-4 majority on the Supreme Court to stop recounts in Florida in the 2000 presidential election, thus enabling Mr Bush to defeat Democrat Al Gore.
However, she upheld the landmark 1973 Roe v Wade decision legalising abortion - the main target of Christian evangelists - and has also defied the religious right on the issue of the separation of church and state.
Within minutes of Justice O'Connor's announcement, activists on both sides - who have been preparing for a Supreme Court showdown for years - launched campaigns to influence Mr Bush's choice.
A left-wing group MoveOn.org began broadcasting television advertisements saying: "Don't nominate an extremist who will undermine the rights of individuals and families as the president did in the Terri Schiavo case several months ago."
Conservative Republican senator Sam Brownback said O'Connor had been a role model for women in the legal profession. As Mr Bush deliberated on a nomination to fill the vacancy, he said, he hoped he would select an individual "who is faithful to the text of the constitution".
Conservatives have complained that the Supreme Court has departed from the constitution in making liberal decisions on gay rights, separation of church and state, and the rolling back of the death penalty.
Republican John McCain said: "I am confident that President Bush will appoint a Supreme Court justice who shares his philosophy, which is a conservative philosophy."
Senator Edward Kennedy, who led the Democratic fight against conservative judges in the past, said: "If it is an in-your-face nomination, it will take time and effort and sweat and tears before the truth is discovered."
In a statement President Bush praised O'Connor as "a discerning and diligent judge", who had earned universal respect.
He said he would consult with senators to find a successor, and he would be "deliberate and thorough" in his search.