Updated

A Democratic senator threatened to filibuster an agreement to extend the USA Patriot Act after congressional negotiators announced Thursday that they had reached a deal on the controversial measure.

"I will do everything I can, including a filibuster, to stop this Patriot Act conference report, which does not include adequate safeguards to protect our constitutional freedoms," said Sen. Russ Feingold, D-Wis., the only senator to vote against the original version of the Patriot Act.

Compromise legislation had been held up in conference since early November over disagreements to reauthorize the anti-terrorism law before it expires on Dec. 31. The compromise must now pass muster with a majority of lawmakers in the House and Senate. If a filibuster were mounted, 60 senators would have to vote to end it before a vote could be held on the legislation.

In the weeks after the Sept. 11, 2001, terrorist attacks Congress passed the anti-terrorism measures to give government more freedom to use surveillance and prosecutorial powers against terror suspects and their supporters.

Thursday's deal keeps three of the provisions most objectionable to civil libertarians — roving wiretaps; secret warrants for viewing business records, books and other items from hospitals and organizations; and "lone wolf" surveillance that allows the U.S. law enforcement agents to monitor terrorists who may act independently of a foreign supporter. The provisions would expire in four years and require congressional action to be reauthorized at that time.

Negotiators backed down from previous demands from the House to extend the three provisions for up to 10 years, opting for the shorter sunset. Most of the Patriot Act would become permanent under the proposed authorization bill.

Senate Majority Leader Bill Frist, who scheduled the bill for a Senate vote next week, said the Patriot Act has led to charges of more than 400 suspected terrorists and the break-up of terrorist cells in New York, California, Oregon, Virginia and Florida. He said he supports the compromise.

"Today's agreement marks a huge step forward in keeping America safe," Frist, R-Tenn., said. "This improved version of the Patriot Act strengthens anti-terrorism protections and enhances safeguards to protect our civil liberties and privacy."

Attorney General Alberto Gonzales said he "applauds" the reauthorization bill, adding that the current form meets administration standards and he would not support additional changes to it.

The White House also gave its backing.

"The Patriot Act is critical to winning the war on terrorism," White House press secretary Scott McClellan said. "The president urges both houses of Congress to act promptly to pass this critical piece of legislation."

But opponents said the deal fails to protect law-abiding Americans.

"We call on all fair-minded lawmakers to reject this hijacked legislation and stand firm against pressure from the administration to compromise on protections in our Bill of Rights," said Caroline Fredrickson, Washington legislative office director for the American Civil Liberties Union.

Feingold said he would fight to get "adequate safeguards to protect" Americans' constitutional freedoms.

"This battle is not over," Feingold said.

Sen. Arlen Specter, R-Pa., chairman of the Senate Judiciary Committee and sponsor of the Senate legislation, said he doesn't think a procedural block would work.

"I don't think there will be a filibuster," said Specter, who announced the deal. "I don't think it will succeed if there is one."

Specter described the measure as balanced — not a "perfect bill but a good bill" brokered between House and Senate negotiators. He added that he wasn't totally satisfied with the revised bill, but it was "a very, very difficult conference" and there has "never been any doubt about the need to reauthorize the Patriot Act."

Sen. Patrick Leahy, D-Vt., the ranking member of the Senate Judiciary Committee and one of the Senate conferees, said he is not signing onto the conference report because he opposes the compromise.

Other Republican Senate conferees have also not yet signed on to support the compromise, sources told FOX News.

Still, Leahy is one of the senators, along with Specter, Sens. Pat Roberts and John Rockefeller, who received a letter from six Senate colleagues saying they oppose the compromise and want changes before they will support it.

"The conference report, in its current form, is unacceptable. ... If further changes are not made, we will work to stop this bill from becoming law," wrote Sens. Feingold, Republicans Larry Craig of Idaho, John Sununu of New Hampshire and Lisa Murkowski of Alaska, and Democrats Dick Durbin of Illinois and Ken Salazar of Colorado.

Click here to read the senators' letter.

Among the changes that were made, the reauthorization specifies that national security letters, an FBI investigative tool that encourages businesses to hand over records on customers without a court order or grand jury subpoena, can be reviewed by a court. It also explicitly allows NSL recipients to inform their lawyers about them.

The Bush administration contends that such consultation already is allowed, citing at least two court challenges to NSLs. However, in a letter obtained by the ACLU under the Freedom of Information Act and posted on its Web site, the FBI prohibits the recipient "from disclosing to any person that the FBI has sought or obtained access to information or records under these provisions."

The six senators who threatened to hold up the legislation said they want more judicial review of gag orders on NSLs and penalties removed on individuals who have received an NSL and violate the gag order. They also want the rule to sunset after four years. Currently, they said, it does not have an expiration date.

They also argued against Section 215 in the Patriot Act. That provision allows the government to obtain "sensitive personal information" that the lawmakers say will allow the government to go on "fishing expeditions."

The senators said they want the compromise bill to require the government to get a judge to approve the relevance of the documents sought. They also want the gag lifted on recipients of subpoenas for sensitive personal information.

Lastly, senators said they oppose the 30-day deadline for the government to notify targets of "sneak and peek" searches and want a seven-day limit.

Rep. James Sensenbrenner, R-Wis., sponsored the House version. The House is expected to vote on the bill on Tuesday, Specter said Sensenbrenner told him.

FOX News' Trish Turner and The Associated Press contributed to this report.