ASHINGTON, Nov. 28 — The growing debate over the Bush administration's antiterrorism initiatives broke onto the public stage today during a Senate hearing where a senior Justice Department official was forced to deal with questions about the planned use of military tribunals and the arrest and interrogation of hundreds of mostly Middle Eastern men.
Senator Patrick J. Leahy, chairman of the Senate Judiciary Committee, said the Bush administration was failing to "respect the checks and balances that make up our constitutional framework."
In responding, Michael Chertoff, the assistant attorney general in charge of the criminal division, told the Senate Judiciary Committee: "Are we being aggressive and hard- nosed? You bet we are. In the aftermath of Sept. 11, how could we not be?"
Mr. Chertoff, a major architect of the antiterrorism program, spoke in gentle tones, but the atmosphere in the Senate hearing room was filled with the political strains and philosophical debates of the last few weeks.
He said the president's actions, including the questioning of thousands of men from Middle Eastern countries, the detention of more than 600 people on immigration and criminal charges, and the monitoring of conversations between lawyers and some defendants, were all intended to prevent future terrorist acts.
"We are pursuing that priority aggressively and systematically with a national and international investigation of unprecedented scope," he said. "But we are carefully doing so within established constitutional and legal limits."
Senator Leahy of Vermont was one of several committee members, mostly Democrats, who sharply criticized the administration's approach. He noted that Congress quickly enacted an antiterrorism bill last month with the cooperation of the administration.
"In the wake of that achievement, the administration has departed from that example to launch a lengthening list of unilateral actions," Mr. Leahy said, referring to the batch of executive branch orders putting new initiatives in place.
Senator Arlen Specter of Pennsylvania was the only Republican to voice displeasure with the administration; he complained that Congress was not consulted about the plans for military tribunals.
"It was surprising to me that the attorney general did not consult with any member of the committee," Mr. Specter said. When Mr. Chertoff began his response by saying that the president and attorney general regard Congress as a "full partner" in the fight against terrorism, Mr. Specter interjected, "How can you talk about full partnership when nobody let us know this executive order was coming down?"
In his testimony, Mr. Chertoff introduced a new explanation for the department's detention of about 600 men, on criminal and immigration charges. He said it was designed, in part, to catch and disable what he called "sleeper" agents Osama bin Laden might have sent to the United States.
"A sleeper is a committed terrorist sent sometimes years in advance into a possible target location, where he may assume a new identity and live an outwardly normal life, all the while waiting to launch a terrorist attack," he said.
Mr. Chertoff said Mr. bin Laden's Al Qaeda organization is known to use sleeper agents. He cited the case of Mohammed Odeh, who was convicted this year for his role in the bombing of the United States Embassy in Nairobi. He said Mr. Odeh lived undercover in Kenya for nearly five years before he was activated.
"Now, how are we going to combat the terrorists' use of sleepers?" Mr. Chertoff said. "We could continue as before and hope for the best, or we can do what we are currently doing — pursuing a comprehensive and systematic investigative approach that uses every available lawful technique to identify, disrupt and if possible, incarcerate or deport persons who pose threats to our national security."
The Justice Department this week disclosed for the first time the names of 93 people held under federal criminal charges, but Mr. Ashcroft said he would not release the names of 548 people arrested on charges of violating immigration regulations.
Mr. Ashcroft told reporters that he could not do so because the law properly prevents the department from creating a public black list of detainees that would violate their rights.
But under questioning today from Senator Russell D. Feingold, Democrat of Wisconsin, Mr. Chertoff acknowledged that there was no specific law blocking the release of the names. Still, he said he believed there was a legitimate concern about violating the privacy of those arrested.
Senator Orrin G. Hatch of Utah, the committee's ranking Republican, defended Mr. Chertoff and Mr. Ashcroft and said that for all the complaints in Congress and among some civil liberties groups, most Americans support the administration's antiterrorism approach.
"Indeed, most Americans worry that we are not doing enough to thwart potential terrorist attacks, not that we are doing too much," Mr. Hatch said.
Mr. Chertoff also defended the executive order that allows the president to have a suspected terrorist tried before a military tribunal that could convict on a two-thirds majority vote of the officer board, impose the death penalty and conduct its proceedings in secret.
He described this as necessary to protect the public.
Senator Mitch McConnell, Republican of Kentucky, said that if terrorists were brought back to the United States, "we'd have the potential for a repeat of the O. J. Simpson trial, complete with grandstanding by defense lawyers."