Republican negotiators accepted a White House-brokered deal yesterday that clears the way for Congress to vote next week on whether to renew the USA Patriot Act's most controversial provisions for four years, in slightly modified forms.

GOP leaders called the development a major breakthrough in a long and contentious debate over whether and how to renew the law, parts of which are set to expire Dec. 31. Since it took effect four years ago, after the Sept. 11, 2001, attacks, the act has made it easier for federal agents to secretly tap phones, obtain library and bank records, and to search the offices or homes of suspected terrorists.

But the agreement faces an uncertain future. No Democratic negotiators in the House or Senate embraced the bill that emerged from the conference committee, and a bipartisan group of senators complained that the proposed revisions do too little to protect the civil liberties of innocent Americans. Proponents had hoped for bipartisan support, but said they believe the bill can survive threatened efforts in the Senate to block it. Some warned, however, that the vote could be close.

The White House intervened this week to coax House Republican leaders to accept a four-year extension of the law's most controversial provisions, rather than a seven- or 10-year extension, as they had indicated they preferred. The concession was enough to win the endorsement of Senate Judiciary Committee Chairman Arlen Specter (R-Pa.), but not of the Democrats on his panel. Specter called the measure "not a perfect bill, but a good bill."

But three Senate Democrats and three Republicans issued a statement saying they are "gravely disappointed" that Specter and others agreed during House-Senate negotiations to drop "modest protections for civil liberties" that were included in a version the Senate passed unanimously this year. They predicted the Senate will reject the compromise bill.

The six were Republican Sens. Larry E. Craig (Idaho), John E. Sununu (N.H.) and Lisa Murkowski (Alaska), and Democrats Richard J. Durbin (Ill.), Ken Salazar (Colo.) and Russell Feingold (Wis.). Feingold vowed to launch a filibuster, which would scuttle extension of the Patriot Act unless 60 of the 100 senators oppose his effort. Some Republicans said Democrats would be foolhardy to block an "anti-terrorism" bill on the eve of an election year.

Also criticizing the bill yesterday were Minority Leader Harry M. Reid (D-Nev.); Patrick J. Leahy (Vt.), the Judiciary Committee's ranking Democrat; and Edward M. Kennedy (D-Mass.).

The compromise bill would slightly change the provisions that allow FBI to obtain people's business records, including library records. Investigators would have to provide a judge with a "statement of facts" showing "reasonable grounds" to believe the records are relevant to an anti-terrorism investigation.

Another provision governs "national security letters," which are used by the FBI to demand customer records from businesses such as telephone companies, Internet providers and libraries. Recipients of such letters are required to keep the requests secret.

The new legislation would explicitly give businesses that receive such letters the right to challenge them in court, but critics say the process is set up in such a way that the government will nearly always prevail. There is also no provision for notifying the individual whose records are being targeted.

As part of the compromise, lawmakers dropped a provision that would have made it a crime punishable by a year in prison to disclose receipt of a national security letter. But the deal retains a five-year prison term if the disclosure is aimed at obstructing an investigation.

Leahy and others strongly oppose provisions instructing judges to presume that federal agents should obtain records unless the targeted person can show that the government acted in bad faith. Kennedy called the targeted person's opportunity to challenge a search "arguably worse than nothing."

The Washington Post reported last month that the FBI issues more than 30,000 national security letters a year, a hundred-fold increase over historic norms. The Justice Department disputed the report but has refused to provide its own tally.

The revised law also would allow agents to surreptitiously search a person's home or business without telling the person for 30 days. The Senate bill called for a seven-day limit on such "sneak and peek" powers; the House version allowed 180 days.

Attorney General Alberto R. Gonzales called the compromise bill a "win for the American people."

The American Civil Liberties Union condemned the agreement, arguing that it will continue to allow the FBI to obtain "a huge array of extremely private records of innocent Americans" with little oversight or limitation.