WASHINGTON, June 27 — Senators on the Judiciary Committee accused President Bush of an "unprecedented" and "astonishing" power grab on Tuesday for making use of a device that gave him the authority to revise or ignore more than 750 laws enacted since he became president.

By using what are known as signing statements, memorandums issued with legislation as he signs it, the president has reserved the right to not enforce any laws he thinks violate the Constitution or national security, or that impair foreign relations.

A lawyer for the White House said that Mr. Bush was only doing his duty to uphold the Constitution. But Senator Arlen Specter, Republican of Pennsylvania and chairman of the Judiciary Committee, characterized the president's actions as a declaration that he "will do as he pleases," without regard to the laws passed by Congress.

"There's a real issue here as to whether the president may, in effect, cherry-pick the provisions he likes and exclude the ones he doesn't like," Mr. Specter said at a hearing.

"Wouldn't it be better, as a matter of comity," he said, "for the president to have come to the Congress and said, 'I'd like to have this in the bill; I'd like to have these exceptions in the bill,' so that we could have considered that?"

Mr. Specter and others are particularly upset that Mr. Bush reserved the right to interpret the torture ban passed overwhelmingly by Congress, as well as Congressional oversight powers in the renewal of the Patriot Act.

Michelle Boardman, a deputy assistant attorney general, said the statements were "not an abuse of power."

Rather, Ms. Boardman said, the president has the responsibility to make sure the Constitution is upheld. He uses signing statements, she argued, to "save" statutes from being found unconstitutional. And he reserves the right, she said, only to raise questions about a law "that could in some unknown future application" be declared unconstitutional.

"It is often not at all the situation that the president doesn't intend to enact the bill," Ms. Boardman said.

The fight over signing statements is part of a continuing battle between Congress and the White House. Mr. Specter and many Democrats have raised objections to the administration's wiretapping of phones without warrants from the court set up to oversee surveillance.

Last month, Mr. Specter accused Vice President Dick Cheney of going behind his back to avoid the Judiciary Committee's oversight of surveillance programs.

"Where will it end?" asked Senator Edward M. Kennedy, Democrat of Massachusetts. "Where does it stop?"

The bills Mr. Bush has reserved the right to revise or ignore include provisions that govern affirmative action programs, protect corporate whistle-blowers, require executive agencies to collect certain statistics, and establish qualifications for executive appointees.

Senators and two law professors before the panel said that if the president objected to a bill, he should use his power to veto it — something he has not done in his six years in office.

Senator Dianne Feinstein, Democrat of California, said the expansion of executive power would be the "lasting legacy" of the Bush administration. "This new use of signing statements is a means to undermine and weaken the law," she said.

What the president is saying, she added, is "Congress, what you do isn't really important; I'm going to do what I want to do."

Ms. Boardman said the president had inserted 110 statements, which senators said applied to 750 statutes, compared with 30 by President Jimmy Carter. The number has increased, she said, but only marginally, and only because national security concerns have increased since the attacks of Sept. 11 and more laws have been passed. She acknowledged that the increase might be construed as "a lack of good communication" with Congress.

But Senator John Cornyn, Republican of Texas, said the committee was making too much of the statements. "It is precedented," he said, "and it's not new."

Senators said they had been expecting a higher-ranking official from the office of legal policy, and Senator Patrick J. Leahy of Vermont, the senior Democrat on the committee, chastised the White House for not sending "anybody who would have authority to speak on this."

"But then, considering the fact that they're using basically an extra-constitutional, extra-judicial step to enhance the power of the president, it's not unusual," he said.