WASHINGTON -- House-Senate negotiators have reached a compromise on extending the USA Patriot Act, Sen. Arlen Specter said yesterday, but the prospects for its approval by the Senate appeared shaky.

A bipartisan group of six senators said it would work to defeat the legislation because the act did not do enough to protect Americans' privacy rights.

As chairman of the Senate Judiciary Committee, Mr. Specter has spent the past two weeks trying to work out a compromise on the anti-terrorism law with House Judiciary Committee Chairman F. James Sensenbrenner Jr., R-Wis. The act expires on Dec. 31.

The Patriot Act, enacted in 2001 following the Sept. 11 terrorist attacks, runs out at year's end unless Congress extends it.

Mr. Specter, R-Pa., was most concerned about putting a four-year expiration date on two of the most controversial items: the ability of the FBI to get approval for roving wiretaps and the so-called library provision, which authorizes the FBI to obtain the library and business records through a court order simply by showing that the records are relevant to a terrorism investigation.

The House version of the legislation put 10-year expiration dates on both provisions, and House Republican negotiators were pushing for seven years as a compromise. But before Thanksgiving, Mr. Specter said he would not sign on to a compromise if the expiration date were more than four years away. He even asked President Bush to intervene on his behalf -- managing to catch him in person by waiting in a long line at a White House ball this week where the president posed for pictures with guests.

Mr. Specter and Mr. Sensenbrenner hammered out a compromise late Tuesday night in which the two most controversial provisions would expire in four years and the FBI would face more requirements for getting wiretaps and library or business records. Most of the Patriot Act's other provisions would become permanent.

"It's a good bill ... not a perfect bill," said Mr. Specter, describing the bill as "balanced." The House may vote on the proposal next week. Mr. Specter said he did not expect a filibuster in the Senate and believed he could gather enough support to pass the compromise.

But other senators clearly do not share that view.

The Judiciary Committee's ranking Democrat, Sen. Patrick J. Leahy of Vermont, who is part of the conference committee, said he could not support the compromise because it did not address Americans' biggests concerns about the act. For example, he said, the legislation doesn't provide adequate judicial review of "national security letters."

Mr. Leahy and the three other Democratic Senate negotiators urged their Republican colleagues to a three-month extension of the act to allow Congress to improve the legislation.

Sen. Russell Feingold, D-Wis., said he would "do everything I can, including a filibuster, to stop this Patriot conference report" because it did not include reasonable privacy protections for Americans.

Mr. Feingold said he appreciated the four-year expiration date on some of the controversial provisions, but added that "merely sunsetting bad law is not adequate."

Proponents of the Patriot Act compromise would need 60 votes in the Senate to defeat a filibuster. Five senators -- Larry E. Craig, R-Idaho; Lisa Murkowski, R-Alaska; John E. Sununu, R-N.H.; Richard J. Durbin, D-Ill., and Ken Salazar, D-Colo. -- joined Mr. Feingold in calling on House colleagues to reject the conference report and take up the Senate version.

"The sunsets this year provide the best opportunity to make the meaningful changes to the Patriot Act that the American public has demanded," the six senators said in their statement. "We believe that this conference report will not be able to get through the Senate, while the Senate bill would easily pass the House if its leadership would bring it to a vote."

The American Civil Liberties Union denounced the House-Senate compromise as a "sham."

But Bush administration officials praised the House-Senate compromise yesterday. Attorney General Alberto R. Gonzales said he had indicated "we would not support changes to the act that would make America less safe."

"I think what's come out of the committee meets that standard," he said.

Under the compromise, the FBI would have to provide more specific information about the specific person who is the target of their wiretap of a phone and must notify the federal tribunal known as the Foreign Intelligence Surveillance Court within 10 days after beginning surveillance of a new phone while providing "facts and circumstances" for taking the action. For business records, the FBI would now have to give the courts a statement of facts showing "reasonable grounds to believe" the records are relevant to a terrorism or espionage investigation.

Mr. Specter said the compromise also addresses some of the concerns about "national security letters," which have been used by the FBI to collect financial or other records from businesses-- such as computer logs showing the Web sites a person visited or telephone logs from a phone company showing who they called.

The Specter-Sensenbrenner compromise would put in new provisions explicitly stating that a business that received such a letter asking for the records of their customers could consult a lawyer and request that a judge review the FBI's request. If the court determined the FBI's request was "unreasonable or oppressive," it could modify or set aside the request.