According to John Yoo, the president's powers under the Constitution are so broad that the Constitution itself cannot restrain them. In a recently declassified 2003 memo, the former Justice Department official asserted that Congress, despite its Article I powers to "make rules concerning captures on land and water" and "for the government and regulation of the land and naval forces," has no business regulating the treatment of military prisoners. Yoo also cited a 2001 memo in which he had concluded that "the Fourth Amendment had no application to domestic military operations."

Compared to Yoo, all three of the remaining major-party candidates for president sound moderate when they talk about executive power. But Barack Obama is the one who seems to care most about restoring the rule of law and the separation of powers after eight years of an administration that has sorely abused both.

Even the Justice Department has backed away from Yoo's maximalist position, although exactly how far isn't clear. In Senate testimony last week, Attorney General Michael Mukasey repeatedly dodged the question of whether he thinks the Pentagon is free to conduct unreasonable searches and seizures.

Such immunity from the Fourth Amendment would allow not just warrantless surveillance of international communications involving people in the United States but monitoring of purely domestic phone calls and email as well. Indeed, it would allow warrantless domestic searches and seizures of any kind, provided they are carried out by a branch of the Defense Department that asserts a connection to terrorism or some other national security threat.

Yet the strongest reassurance Mukasey could offer was to say that "the Fourth Amendment applies across the board, regardless of whether we're in wartime or in peacetime." Asked specifically whether that means it applies to "domestic military operations," he said, "I'm unaware of any domestic military operations being carried out today."

Mukasey's evasiveness is especially troubling in light of his refusal during his confirmation hearings to acknowledge that Congress has the constitutional authority to restrict National Security Agency wiretaps. Unlike Yoo, he did at least concede that the president is bound to obey a congressional ban on torture.

That is the area where John McCain has most clearly distinguished himself from the Bush administration. Last December, in response to a Boston Globe candidate survey focusing on executive power, the Arizona senator also said the president is not free to violate statutory restrictions on wiretaps, and he rejected the use of signing statements as a way of reserving the right to flout laws. But he took a broader view than the other candidates of the president's authority to detain "enemy combatants," and he declined to identify areas where the Bush administration has overstepped its constitutional authority.

Obama, by contrast, gave half a dozen detailed examples. In general, the Illinois senator's answers to the Globe's questions were direct, thoughtful and complete, apparently reflecting a sincere determination to limit his own power if elected.

After the election, of course, such promises may not be worth much. But on that score I worry more about Hillary Clinton. The New York senator's answers to the Globe survey, though less detailed than Obama's, were similar in substance. I just find it hard to believe them.

Clinton agreed, for example, that the president has to seek congressional authorization before attacking another country, except in response to an "imminent threat." Yet she has bragged about urging her husband to bomb Serbia as part of an unauthorized war that had nothing to do with national defense.

Although Clinton now claims to have a modest view of presidential power, she was singing a different tune a few years ago. "I'm a strong believer in executive authority," she told George Stephanopoulos of ABC News in 2003. "I wish that, when my husband was president, people in Congress had been more willing to recognize presidential authority." With the War on Terror as a rationale, her wish could be her command.