The formal constitutional rules for appointing a justice to the Supreme Court seem simple enough: The president "nominates" a candidate and the Senate gives its "advice and consent." But beneath this terse constitutional text lie other norms implicit in constitutional structure and embodied in long-standing practice. These rules will define fair and foul play in the weeks ahead as the nation witnesses its first Supreme Court appointment in a decade.

While the Constitution's text does not say whether a president may consider a would-be judge's ideology, the very nature of the appointments process invites such consideration. The Founders intentionally politicized judicial appointments by vesting these appointments in elected executive and legislative officials. In several modern European countries, by contrast, many judges are selected by fellow judges in bureaucratic, civil service fashion.

Though no judicial nominee should ever publicly or privately promise a particular judicial outcome, a president may properly try to predict how a nominee is likely to behave, and presidents have done so from the beginning. Each of the eight men who sat on the Supreme Court before 1796 had been a prominent Federalist ally of George Washington in the struggle to ratify the Constitution. The first former anti-Federalist to serve, Samuel Chase, did not win Washington's approval until after Chase had shown himself to be a strong supporter of the Washington administration. Federalist John Adams tried to pack the judiciary with Federalists, Republican Thomas Jefferson with Republicans. Not until Republican Abraham Lincoln named Democrat Stephen J. Field did a president openly cross party lines in a Supreme Court nomination, and when Lincoln did so in 1863, the deepest ideological divide ran not between Republicans and Democrats but between Unionists and Secessionists.

What goes for the president also goes for senators; they, too, may openly consider ideology in appointing judges. This tradition also stretches back to the nation's founding, when senators in 1795 rejected John Rutledge for chief justice because they doubted his political judgment.

Although constitutional text establishes the same advice-and-consent framework for Cabinet posts and Supreme Court positions, Senate practice has long treated the two differently. Cabinet officers are executive branch officials who answer to the president and will leave office when he leaves. Justices are life-tenured non-executive officials in an independent branch. Because they may remain in office long after the president's term expires, and are emphatically not part of the president's team, the Senate has rightly given much less deference to presidents' judicial nominations. By 1830, the Senate had defeated three Supreme Court nominations but had yet to say no to any of the much larger number of Cabinet candidates.

In the give and take between the president and the Senate, the executive has the upper hand. Though the document speaks of senatorial "advice," only the president makes actual nominations, and once this happens, it is hard for the Senate to say no. A president is always free to name his first choice over his third, while a senator who says no to her own third choice has no guarantee that the president will ever nominate one of the senator's top two picks. In essence, a senator must vote down an actual person with no assurance that the unknown alternative behind Door No. 3 will be any better. Whereas the president need only make up his own mind, there may in fact be no single nominee who heads the wish list of a majority of senators.

Also, voting against a flesh-and-blood nominee with friends and family, and perhaps a compelling life story, is much harder than voting against an ordinary bill sponsored by the administration. While nothing in the Constitution's text or history gives a nominee an absolute right to an up-or-down floor vote, basic notions of fair play make it hard to deny a high-visibility candidate such a vote.

Further presidential advantages appear if two vacancies arise simultaneously. (Many pundits have predicted that Chief Justice William H. Rehnquist will step down a little later this summer.) Even hard-core Democrats may find it difficult to oppose two nominations simultaneously -- the appointments equivalent of a two-front war -- and the president may use one nominee to draw fire away from another.

Thus, if President Bush seeks to win the appointments game at any cost, the odds favor him. But the political nature of judicial appointments reminds us that there is another possible appointments strategy that may better serve the president's long-term self-interest. By naming someone whom even moderate Democrats could cheerfully support, the president could redeem his claim to be a uniter rather than a divider -- a claim that might enable Republicans to hold their ground or even pick up seats in the Senate elections of 2006. Such gains would not only bolster the Bush agenda in obvious political ways, but they also could even help the president on the court itself, given that there could well be yet another Supreme Court vacancy in 2007-08. The more Senate seats Bush controls then, the more running room he will have. It's worth remembering, for example, that Justice John Paul Stevens is 85.

The appointments season is perhaps only just beginning.

The writer teaches constitutional law at Yale University and is the author of the forthcoming book "America's Constitution: A Biography."