WASHINGTON – House and Senate negotiators reached an agreement Thursday to extend the USA Patriot Act, the government’s premier anti-terrorism law, before it expires at the end of the month. But a Democratic senator threatened a filibuster to block the compromise.

“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., who was the only senator to vote against the original version of the Patriot Act.

Senate Judiciary Chairman Arlen Specter, R-Pa., announced that the negotiating committee had reached an agreement that would extend for four years two of the Patriot Act’s most controversial provisions – authorizing roving wiretaps and permitting secret warrants for books, records and other items from businesses, hospitals and organizations such as libraries. Those provisions would expire in four years unless Congress acted on them again.

“All factors considered it’s reasonably good, not perfect, but it’s acceptable,” Specter said of the agreement.

Also to be extended for four years are standards for monitoring “lone wolf” terrorists who may be operating independent of a foreign agent or power. While not part of the Patriot Act, officials considered that along with the Patriot Act provisions.

The Republican-controlled House had been pushing for those provisions to stay in effect as long as a decade, but negotiators decided to go with the GOP-controlled Senate’s suggestion.

Most of the Patriot Act would become permanent under the reauthorization.

The White House applauded the agreement.

“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.”

Democratic Leader Harry Reid of Nevada intends to vote against the measure as currently drafted, according to an aide.

Feingold and five other senators from both parties issued a statement that said, “We believe this conference report will not be able to get through the Senate.” They said they wouldn’t support it in any form.

The other senators are 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.

Feingold issued a separate statement threatening a filibuster, a stalling technique designed to block the measure from coming to a final vote.

It takes 60 senators to overcome a filibuster in the 100-member Senate.

“I don’t think there will be a filibuster,” Specter said. “I don’t think it will succeed if there is one.”