There are few genuine earthquakes in American politics, but yesterday's announcement by Justice Sandra Day O'Connor easily qualifies as one. Her retirement is likely to trigger one of the most consequential confirmation battles in a generation, with the ideological balance of the Supreme Court and the future of contentious social issues now firmly in the hands of President Bush.

For more than three decades, the right has been ascendant in America. A Republican Party fueled by the energy and activism of its religious and social conservatives has seized control of the executive and legislative branches of government in Washington and has seen its strength widen and deepen across the country.

The Supreme Court is the lone branch of government where conservatives have been unable to gain the dominant voice, to the great frustration of those on the right. Among the most ardent conservatives, there is no better evidence of the need to change the court than two decisions that marked the end of the term this summer, which limited displays of the Ten Commandments on government property and gave local governments the power to seize private property for commercial development.

The choice ahead for Bush in selecting a successor to O'Connor may prove to be the most important domestic decision of his presidency, given its potential impact on abortion and other issues and rivaling Iraq in its ability to split the country. He will soon decide just how far to try to push the court in a different direction. The bolder he is, the more likely he will touch off a battle that will consume Washington and much of the country for the rest of the summer and whose impact could be felt in the 2006 elections and beyond.

There are huge risks for Bush no matter which way he moves. He faces enormous pressure from the right to appoint someone more conservative than O'Connor. But such a move risks a potentially bruising battle with Senate Democrats and a backlash among voters in the middle of the ideological spectrum, who may worry about the GOP's social and cultural agenda.

But leading Republicans said Bush must move decisively to remake the court. "Given the disastrous decisions of the last few days on private property rights, the Ten Commandments and violent criminals, I think the majority that elected Bush and the House and Senate clearly expects a very conservative nomination," said former House speaker Newt Gingrich of Georgia.

The coming decision will prove a defining moment for Bush's presidency. His political strategy is founded on the assiduous courtship of his conservative base, and on big issues he has rarely chosen to disappoint them. But it has never been clear how comfortable the president is in pushing a sharply conservative social agenda. He opposes abortion but has resisted championing the cause of overturning Roe v. Wade. He pushed a constitutional amendment barring same-sex marriages, but at a time when it was clear it could never pass Congress.

Bush has been forthright, however, in trying to change the complexion of the courts, a political project that he shares with White House Deputy Chief of Staff Karl Rove and other top advisers. Bush has nominated a series of conservatives to fill federal district and, especially, appellate courts, locking horns with Democrats, whose filibusters of those nominations only heightened the White House's determination to keep pushing forward.

But the president also has a contrarian streak, and in personnel decisions he often likes to go with his instincts. Only a handful of people know the full range of calculations he and his advisers are bringing to this decision, but there is no more important issue to social and religious conservatives than the Supreme Court, and there aren't many ways for Bush to fudge on his intentions in the selection of a successor to O'Connor.

"The court is the pivot point on social policy, and O'Connor's seat is the pivot point on the court," said William Kristol, editor of the conservative Weekly Standard, who had predicted O'Connor's retirement. "So it's a pretty dramatic moment."

Both sides believe the political landscape tilts in their direction in a Supreme Court battle, reflecting sharply contrasting philosophies about the role of the courts.

Democrats think Bush has been hurt by Republican-led intervention in the Terri Schiavo case and by the comments of House Majority Leader Tom DeLay (R-Tex.) and others warning that they intend to do whatever they can to rein in the courts. In their estimation, moderate voters, particularly women, recoil at the prospect of a Republican Party that already controls the White House and Congress gaining control of the majority at the Supreme Court. Instead, they want a court system that provides a check and balance rather than a ratification of the conservative agenda.

But conservatives argue, and key White House officials agree, that the public sees the courts as out of touch with their values. They believe that voters want the courts to act as impartial referees in the political battles between the major parties. Conservatives believe a majority of Americans see the courts dominated by judges who are legislating on issues such as gay rights and abortion. They argue that the high court's rulings run wildly counter to public opinion.

The shockwaves of O'Connor's announcement began to reverberate immediately as word began to circulate yesterday morning that it was the court's most celebrated swing justice, and not the ailing chief justice, William H. Rehnquist, who is stepping down.

Combatants on both sides had widely anticipated a Rehnquist retirement and were gearing up for a confirmation fight over his successor. But there was something ritualistic about the preparations for that battle, since the departure of Rehnquist, one of the staunchest conservatives among the nine justices, would offer no real chance to move the court to the right.

The departure of O'Connor changed all that, bringing conservatives to a moment they have long dreamed about and liberals to a moment they have long feared. No one was exaggerating yesterday when they talked about the potential consequences involved in the choice of an O'Connor successor.

"This totally raises the stakes," said Democratic pollster Geoffrey Garin. He added: "Obviously we've had battles over particular nominees, but this is a battle about the court itself rather than any one seat. As a result, so much more hangs in the balance. There's no question that all sides will see this as a very high-stakes, high-consequence battle."

It is possible that predictions of a pitched battle in the Senate this summer may prove exaggerated. "I understand both sides are well armed and ready for war and everyone's got their battle plans drawn," said Ron Klain, who was chief of staff to then-Vice President Al Gore. "But it ultimately does come down to whom he selects. If he does what Ronald Reagan did and veers off and picks someone more from the center, then we might not have a fight. If not, we're much more likely to have a fight."

That could mean the choice of someone such as Attorney General Alberto R. Gonzales, a personal favorite of the president, who is seen as less rigidly conservative than some others on lists of possible nominees.

But among conservatives still angry about the choice by Bush's father of Justice David H. Souter, who has voted regularly with the court's liberals, Gonzales is seen as insufficiently opposed to abortion, and his nomination would be viewed by them as a betrayal. "There are many pro-family groups who are opposed to his being nominated," said Jan LaRue, chief counsel of Concerned Women for America.

Depending on the outcome of the upcoming battle, the courts could become central issues in next year's midterm elections and in the next presidential campaign. After the uproar over the confirmation of Justice Clarence Thomas came what was called the "Year of the Woman," in 1992, when Democrats elected a number of women to the Senate. After the high court's 1989 abortion decision in the Webster case from Missouri, abortion became a mobilizing issue on both the right and the left.

In the larger context, conservatives see the upcoming battle as a chance to recoup from the failure to confirm Robert H. Bork to the Supreme Court during the Reagan administration, which they regard as a major setback for their movement. In Bush they have the most conservative president since Reagan and one who, as his advisers say, likes to swing for the fences. Which is why his next move is awaited by all sides with such great expectation.

Sandra Day O'Connor, appointed by President Ronald Reagan, walks down the Supreme Court steps with Chief Justice Warren E. Burger at her 1981 swearing-in.President Bush walks to a Rose Garden news briefing on the retirement of Supreme Court Justice Sandra Day O'Connor. Bush's selection of a successor amid pressure from the right and the left may prove to be the most important domestic decision of his presidency.