Nobody can predict the future shape of the constitutional world that was created on September 11, but history can put it in perspective. This is not the first time American leaders have attempted a radical reorganization of the nation's aims and ambitions. From the country's founding through the civil-rights revolution, there have been many similar efforts -- sometimes successful, sometimes disastrous. What is distinctive about this one?

Most successful transformations have been the product of political movements that gained grass-roots support for decades before their final triumph. An entire generation's struggle to redefine America lies behind the Constitution of 1787, and the same is true of the Reconstruction Amendments of the 1860s and the Civil Rights Acts of the 1960s. But this isn't true of the new politics emerging out of 9-11. Rather than responding to a movement from within, we are responding to hostile forces from without. Pearl Harbor provides the most obvious parallel. But World War II came to a decisive end, while the "war on terrorism" threatens to be both endless and more consequential in its constitutional implications.

A better analogy is the Great Depression of 1929. When Herbert Hoover beat Democratic nominee Al Smith in 1928, neither had a clue about the great challenges ahead -- and it took a decade before the country reached a new constitutional equilibrium. Hoover's presidency provided an essential part of the learning process. Rather than being the "do nothing" president of historical myth, Hoover in fact desperately tried to fight the Depression -- just not at the cost of smashing the existing constitutional framework of limited government. It was only after this effort at adaptation failed that the country was ready for Franklin Delano Roosevelt's effort to revolutionize constitutional understandings. The New Deal generated a sustained crisis of legality that came to an end only when the president had won a landslide re-election and threatened the Supreme Court in his famous court-packing plan of 1937.

George W. Bush is no Hoover. He is not responding to the new threat by governing within the existing legal framework. His war against terrorism is generating a crisis of legality of Rooseveltian proportions. He has asserted the authority to consign American citizens to prison indefinitely without due process by declaring them "enemy combatants." He has claimed the right to imprison hundreds of inmates at Guantanamo Bay without the individualized hearings required by the Geneva Convention. And he has invaded Iraq in violation of the United Nations Charter. Because the U.S. Senate ratified both the Geneva Convention and the UN Charter as treaties, the U.S. Constitution expressly makes them part of "the supreme law of the land." In violating these treaties, the president is not merely violating international law; he is violating American law, despite his constitutional charge "to take care that the laws be faithfully executed."

But in contrast to Roosevelt's administration, this break from existing legal frameworks has not followed an earlier period of more modest presidential adaptation. Moreover, the courts may play a lesser role in checking the Bush administration's transformative ambitions than they did during Roosevelt's administration. Though the Iraq War may violate America's treaty commitments to the United Nations, Congress last fall expressly authorized the president's decision. Under well-established case law, this means that American courts will refuse to consider any lawsuit based on the UN Charter.

Challenges to other presidential actions are currently percolating in the lower courts, but -- with one or more Supreme Court justices possibly planning to resign in the next 18 months -- the High Court's willingness to review such cases could weaken if Bush is able to stock it with still more conservative justices. The Senate should therefore be prepared to scrutinize the president's nominations with the greatest care. As Roosevelt was, Bush should be required to return to the people for re-election before he can claim a mandate to revolutionize constitutional doctrine. The burden should be on the administration to prove that its nominees will not sacrifice fundamental civil liberties in the name of an endless war on terrorism. Unless nominees come to the Senate with established track records of fidelity to civil rights, Senate filibusters would be entirely appropriate.

Whatever happens in the short term, civil libertarians should be under no illusions. If Republicans sweep in 2004, Bush will follow Roosevelt and use his second term to transform the Supreme Court with judicial partisans of the newly ascendant orthodoxy. In the 2000 campaign, Al Gore failed to convince the American people that there were large differences between him and George Bush. But nobody should doubt that the next campaign will be among the most fateful in American history.

Bruce Ackerman, a professor of law and political science at Yale University, is the author of We the People.