Approval came on a voice vote late Wednesday night, and cleared the way for a final vote in the House.
The agreement capped several days of back-room negotiation conducted against the backdrop of presidential attacks on critics of the legislation. The Patriot Act provisions will expire Dec. 31 if the House and Senate do not act.
The extension gives critics — who successfully filibustered a House-Senate compromise that would have made most of the law permanent — more time to seek civil liberty safeguards in the law. Democrats and their allies had originally asked for a three-month extension, and the Senate's Republican majority had offered a one-year extension. The final deal split the difference.
"For a lot of reasons, it made the most sense, given that there are significant differences that remain," said GOP Sen. John Sununu of New Hampshire, one of a small group of Republicans who joined with Senate Democrats to filibuster a House-Senate compromise.
"I think this is a reasonable conclusion," said Sen. Patrick Leahy of Vermont, the ranking Democrat on the Senate Judiciary Committee.
Republicans who had pushed for legislation that would make most of the expiring provisions permanent said the agreement only postpones the ongoing arguments over the Patriot Act for six months. "We'll be right back where we are right now," said a clearly frustrated Sen. Orrin Hatch, R-Utah.
Sen. John Cornyn, R-Texas, added, "Our intelligence and law enforcement officials should not be left wondering, yet again, whether the Congress will manage to agree to reauthorize the tools that protect our nation."
The bill's critics gained momentum Wednesday when they released a letter crafted by Sununu and Sen. Charles Schumer, D-N.Y., showing they had 52 senators agreeing to support a three-month extension.
"This is the right thing to do for the country," Schumer said after the deal had been announced. "To let the Patriot Act lapse would have been a dereliction of duty."
President Bush, Attorney General Alberto Gonzales and Republican congressional leaders have lobbied fiercely to get the House-Senate compromise passed, and issued dire warnings of what would happen if the Patriot Act expires.
Despite the president's push to renew key elements of the law, many remain
Most of the Patriot Act — which expanded the government's surveillance and prosecutorial powers against suspected terrorists, their associates and financiers — was made permanent when Congress overwhelmingly passed it after the Sept. 11 terrorist attacks on New York City and Washington.
Making permanent the rest of the Patriot Act powers, like the roving wiretaps which allow investigators to listen in on any telephone and tap any computer they think a target might use, has been a priority of the Bush administration and Republican lawmakers.
The battle capped off a bitter day on Capitol Hill, during which lawmakers squabbled over drilling in the Arctic National Wildlife Refuge and budget cuts.
CBS News politcal analyst Craig Crawford says both sides can claim victories, but only in smaller battles.
"I would say Republicans are getting some of their agenda through," Crawford says, "but they've left an awful lot on the table, going into an election year.
"I would say the Democrats are the winner, except Democrats lack two things: a voice and a message. They have a lot to gain from the Republican in fighting, but Democrats have a long way to go to get some personalities out there on the national stage who articulate a consistent agenda."