WASHINGTON -- The House is expected to approve a House-Senate compromise today reauthorizing and modifying the USA Patriot Act, which expires Dec. 31.

But the legislation faces mounting Senate opposition that could prevent Congress from making it law by the end of this year, and the potential impasse has put Senate Judiciary Chairman Arlen Specter in an awkward position.

As one of top negotiators of the House-Senate compromise, Mr. Specter, R-Pa., spent much of yesterday defending the bill on the Senate floor, even though he acknowledges that it does not go as far as he hoped in protecting civil liberties.

Last summer, Mr. Specter managed to win unanimous support for the bill's Senate version, which included more civil liberties protections. But now, a number of Republicans are actively working with Democrats to marshal the 40 votes needed to sustain a filibuster against the House-Senate compromise.

Sen. Russell D. Feingold, D-Wis., the only senator to oppose the Patriot Act when it was approved to bolster law enforcement powers after the Sept. 11, 2001, domestic terror attacks, yesterday described the reauthorization bill as "essentially nothing. It's going back to square one; it's going back to the mistakes that were made after 9/11."

Mr. Feingold's efforts have been bolstered this week by conservative Republican allies, including Sen. John E. Sununu of New Hampshire, who are helping to pressure fellow senators to defeat the compromise bill.

Even Mr. Specter's initial co-sponsor -- the Judiciary Committee's ranking Democrat, Vermont Sen. Patrick J. Leahy -- has criticized the compromise. Mr. Leahy is urging colleagues to support a three-month extension of the law instead, a move intended to send House-Senate negotiators back to the drafting table.

Senators opposing the bill are also being backed by groups as disparate as Gun Owners of America, the American Civil Liberties Union, and the Association of American Physicians and Surgeons. The groups' representatives yesterday met on Capitol Hill to warn those lawmakers who are up for re-election that they could face consequences if they support the compromise.

But the Bush administration is making a major push for the reauthorization. Attorney General Alberto Gonzales appeared at the Capitol yesterday with House Judiciary Committee Chairman F. James Sensenbrenner Jr., R-Wis., to press for passage. Mr. Gonzales said "the tools" included in the reauthorization are "very important to the success of the Department of Justice in protecting this country."

Mr. Sensenbrenner said he did not consider the proposed three-month extension of the Patriot Act to be "a feasible alternative," a view shared by Senate Majority Leader Bill Frist, R-Tenn.

The compromise makes most provisions of the Patriot Act permanent but would make limited changes to some of its more controversial provisions. Mr. Specter insisted on a four-year expiration date for two provisions: government investigators' authorization to use "roving wiretaps" to monitor suspected terrorists who may be using multiple phones and a provision allowing the FBI to seek court orders to obtain business records that could be related to terrorist activity.

Mr. Feingold, Mr. Sununu and their Senate allies are arguing for more substantial changes to the government's powers to obtain business and library records and government use of "national security letters" -- issued by the FBI to demand information such as phone logs or financial information from businesses or computer network administrators -- about any American who could be linked to a terrorism investigation.

One noteworthy change in the bill would give Americans who receive those "national security letters" the right to ask a judge to quash the FBI request or to modify it. But Mr. Sununu emphasized yesterday that any person asking a judge to throw out the FBI request requires "a showing of bad faith on the part of government. What individual, what business will be able to ever meet that threshold?"

Mr. Frist said he was aiming for a Senate vote later this week. A spokesman for Sen. Rick Santorum, R-Pa., said the senator was likely to support the Patriot Act reauthorization compromise because it includes a provision that is aimed at cracking down on methamphetamine use.