The sums involved, which threaten to rival the most expensive re-election races to the Senate, are a measure of the passions raised by a struggle which has electrified Washington, whose outcome could set the ideological slant of the US high court for a generation.
Two other justices, including the ailing chief justice, William Rehnquist, are likely to step down in the next year or two. But Ms O'Connor was the critical swing vote in a court whose nine members often split five to four. Her replacement thus represents the most important domestic decision President George Bush is likely to take during his eight years in office.
Mr Bush has indicated he will not announce a nominee until after he returns from the G8 summit in Scotland - but in today's culturally polarised America, left and right have been at undeclared war for months. Ads that could belong in the most bitter political campaign proliferate on cable television channels, while the interest groups have set up war rooms to rival a presidential campaign.
The spending is being focussed on the Senate, whose 100 members will vote on any nominee. With one right-wing group - Progress for America - alone planning to spend $18m on television advertising, analysts say that spending is certain to reach $50m, and could approach double that if Mr Bush picks a particularly contentious nominee.
The mechanics are those of a political campaign. Right-wing groups, who urge the President to go for a conservative opposed to abortion and affirmative action, are concentrating their fire on Democratic senators running for re-election in 2006 in states that voted Republican in 2004 presidential vote.
Their liberal counterpart, a loose coalition stretching from trade unions and trial lawyers to gay, abortion and civil rights groups, is doing the reverse, zeroing in on Republican senators - many of them moderates anyway - who must fight for re-election in states won by John Kerry last November.
The message could hardly be plainer. The senator in question should vote as they wish - or risk facing an even larger tide of hostile spending when they come up for re-election. The one chance of averting such a showdown would be for Mr Bush to nominate a candidate acceptable to both sides. Not only, however, is it debatable whether such a person exists, Mr Bush is not one to seek compromise, except on his terms.
He has promised to "consult" the interested parties, including key Senate Democrats, but almost everyone expects him to choose a conservative. Mr Bush has already identified Clarence Thomas and Antonin Scalia - the two most conservative members of the current court - as models for his own nominees.
But the President does not only have Democratic opposition to contend with.
Some Christian conservative groups, for example, have let it be known they would oppose Alberto Gonzales, the current Attorney General and a widely tipped potential nominee. Mr Gonzales is a longtime friend of Mr Bush, and if confirmed would be the first Hispanic member of the court - thus fulfilling a declared ambition of the President. But conservatives have been alarmed by his pro-abortion voting record as a member of the Texas supreme court.
The case underlines the dilemma facing Mr Bush. A comparatively moderate nominee such as Mr Gonzales would probably be spared a bruising confirmation battle in the Senate. But Mr Bush would incur the wrath of social conservatives, his most important electoral constituency.Reuse content