The effort to fill the Supreme Court seat being vacated by Justice Sandra Day O'Connor has already become a fundraising magnet for both left and right that promises to rival the 2004 presidential campaign for the rate of cash flow, if not total dollars raised.

The prospect of shifting the Supreme Court to the right has fueled a quest for dollars by conservative and liberal interest groups that will halt only if President Bush does the unexpected and nominates someone acceptable to all sides.

Under the scenario of an ideological battle, participants predict the competition for cash will turn the Senate confirmation into the most expensive nomination fight in the nation's history, certain to break $50 million and, if the nominee is especially controversial, likely to approach $100 million.

Most of the money raised would not be publicly reported. With the exception of such groups as MoveOn PAC, many organizations active in the fight are tax-exempt and have few, if any, disclosure requirements.

The nomination process will pit two lobbying and interest-group coalitions that have repeatedly gone head to head during the Bush administration over tax cuts, energy legislation, and class-action and bankruptcy measures. While the business-social-conservative coalition has repeatedly defeated the liberal-labor alliance, the outcome of a far more visible nomination fight would be highly unpredictable.

On the right, Progress for America, which spent $45 million in behalf of the Bush 2004 campaign, is committed to spending $18 million for targeted radio and television ads; the allied Judicial Confirmation Network plans to spend $2 million to $3 million for grass-roots lobbying of key senators; and Tony Perkins, president of the Family Research Council, has vowed to put unspecified "millions" behind a conservative nominee.

"The focus of Progress for America is crystal clear," said spokeswoman Jessica Boulanger. "That is to defend the president's nominee from attacks from the left."

The spending by these groups only touches the surface of the expected investment in a conservative nominee by James Dobson's Focus on the Family, by Jay Sekulow's American Center for Law and Justice, by the hundreds of programs broadcast on Christian radio, and by the vast network of other organizations, including the Christian Coalition, the Free Congress Foundation, the Southern Baptist Ethics and Religious Liberty Commission, Concerned Women for America, and business groups such as the National Association of Manufacturers.

On the left, such groups as People for the American Way and MoveOn PAC already flexed their muscles in the debate over the "nuclear option" proposal of Senate Majority Leader Bill Frist (R-Tenn.) to end filibusters of judicial nominees.

With far lower levels of public interest, People for the American Way was able to collect $5 million for the filibuster issue, MoveOn raised $1.5 million and the Alliance for Justice raised $1 million.

"The $1.5 million was just a warm-up for what's to come," said Ben Brandzel, MoveOn's advocacy director. "That was a preparatory round, with far less attention and little focus on the threat to our rights, and $1.5 million was no sweat."

Just as there are hundreds of groups on the right prepared to spend money and time on the nomination, there are a host of counterparts on the left, including the AFL-CIO and its member unions, civil rights organizations, gay and lesbian groups, environmental lobbies, and the well-heeled trial lawyer bar.

Abortion rights groups are expecting the nomination fight to help them regain vitality. With Roe v. Wade on the books for 32 years, many younger women are relatively unaware of the fact that abortion was illegal in most of the United States, liberal women's rights leaders say.

"NARAL Pro-Choice America surpassed its fundraising goals in the hours following Justice O'Connor's announcement," said President Nancy Keenan. Donors "are deeply concerned that President Bush will choose to further divide this nation by nominating a radical right-wing conservative."

Moderation is not the tone of fundraising appeals in the nomination contest. "This is big, people. Huge," NARAL wrote to supporters. "It's true, there is no freedom without choice. Without choice, we are not free."

In private, leaders of organizations on both the left and the right acknowledge that the more controversial the nominee Bush selects, the more money each side will raise.

The money will finance lobbying campaigns conducted in the halls of Congress, on radio and television, in senators' home-state town meetings, in pressure from business and civil rights leaders -- at every leverage point the two sides can find.

The left and right have already designated target senators and put in place the tools of political warfare to mobilize troops at the grass-roots level. Progress for America, for example, has targeted 21 states, including all the states that cast majorities for Bush and have a Democratic Senate incumbent up for reelection in 2006.

In addition, virtually every group is investing in the states of Republican senators who prevented adoption of the nuclear option ending judicial filibusters and in those of Republican moderates who might vote against a very conservative nominee. These key states include Maine, Pennsylvania, Ohio, Arizona and Rhode Island.