The departure of Ms O'Connor, 75, marks the first change to the Court since 1994, an 11-year era of stability without precedent since the early 19th century. Nominated by Ronald Reagan in 1981, she has been the vital swing vote among the nine Justices. With all but one of the justices well above retirement age, the political parties and a host of interest groups have been gearing up for months for a nomination battle. Nonetheless the news came as a bombshell in Washington.
The expectation had been that the first to step down would be William Rehnquist, the Chief Justice, who is 80 years old, increasingly frail and suffering from thyroid cancer. Instead it is Ms O'Connor, the fulcrum of a Court that frequently splits between right and left.
She has cast the decisive vote in a host of 5-4 decisions on issues ranging from abortion and gay rights to affirmative action, in an era when the Supreme Court, not Congress, has refereed the culture wars that divide America. The same happened in the December 2000 ruling that gave George Bush the Presidency.
Replacing Mr Rehnquist with another unwavering conservative would little change the court's ideological make-up. Now however Mr Bush has an opportunity to reshape it, by appointing an outright conservative to succeed the moderate and pragmatic Ms O'Connor. "This is much bigger news than if Rehnquist had gone," David Garrow, a Supreme Court historian said.
Ms O'Connor has been one of the most admired women in America, rising from a modest background in Texas and Arizona to become one of the country's most respected jurists.
Yesterday Mr Bush paid tribute to "a public servant of great integrity" saying her "intellect, wisdom and decency" had inspired all Americans. He promised to be "deliberate and thorough" in his search for a successor, and consult with the legal specialists and key Senators on Capitol Hill.
The White House has long had ready a short list of potential candidates, and some have been interviewed. They are believed to include Alberto Gonzales, a former White House counsel, now Attorney General. He would fulfil Mr Bush's ambition to appoint the first Hispanic to the court.
A moderate conservative like Mr Gonzales would win speedy approval from the Senate. But Mr Bush has expressed admiration for the two most conservative justices Clarence Thomas and Antonin Scalia.
If he follows his instincts and goes for a right-wing nominee, the battle could eclipse in ferocity and bitterness the hearings of 1991 when Mr Thomas was ultimately confirmed - but only after complaining that the procedure was like a "high-tech lynching of an uppity black".
Mr Bush said he would announce his choice soon, so the new judge could be confirmed by the time the Court's new term begins in autumn. Mindful of the fracas over Mr Thomas, the President appealed for a "dignified process of confirmation". But whether his wish is granted depends not only on his choice, but also on the mood of a partisan Capitol Hill. Vicious battles over a handful of Bush nominees to the federal appeals courts have already led to Democratic filibusters and the threat by the Senate's Republican leadership to retaliate with the "nuclear option" of changing the rules so that a candidate could be approved by a majority of 51, instead of 60.
That showdown, which would paralyse the legislature, has been averted by a deal made by a "Gang of 14" Republican and Democrat centrists. But the deal may not survive the supercharged climate of a Supreme Court confirmation battle.
John McCain, an architect of that arrangement, sounded moderately hopeful yesterday. "It won't be a day at the beach, but I'm guardedly confident it won't be a filibuster situation," he said. The O'Connor resignation however may be only the beginning. Mr Rehnquist is still expected to step down in the next year, while another Justice, John Paul Stevens, is even older, at 85.
Next in line?
* JUDGE EMILIO GARZA
Potential nominee in 1991 to the Supreme Court seat that went to Clarence Thomas. Liberals fear that he could vote to overturn the 1973 ruling that legalised abortion. Popular with fellow Hispanics.
*ATTORNEY GENERAL ALBERTO GONZALES
A Bush favourite but maybe not conservative enough for the Supreme Court, particularly on abortion. Helped form policies linked to Guantanamo Bay torture.
* JUDGE MICHAEL LUTTIG
Appointed in 1991 to the very conservative 4th US Circuit Court of Appeals in Richmond, Virginia. His father was killed by a carjacker in Texas.
* JUDGE JOHN ROBERTS
Joined the US Circuit Court of Appeals for DC in May 2003 after a fight in the Senate. Has a (contested) reputation for being a moderate conservative.Reuse content