By a substantial margin, the Senate on Friday passed an extension of a controversial overseas wiretapping program, the Foreign Intelligence Surveillance Act.
The law has already been reauthorized by the House, and President Obama is expected to sign it.
According to Sen. Dianne Feinstein, D-California, who is chairman of the Senate Intelligence Committee, the law, known by its acronym FISA, has helped authorities prevent terrorist plots.
"In the past four years, there have been one hundred arrests to prevent something from happening in the United States," Feinstein said. "Some of these plots have been thwarted because of this program. I think it is a vital program."
But critics say the law gives intelligence agencies wide latitude in carrying out surveillance, under oversight by a special FISA court whose proceedings are not public.
"It's something with enormous potential for abuse," said Julian Sanchez, with the libertarian Cato Institute. "The FISA Amendments Act authorizes what the Fourth Amendment was written to prevent, which is to say, general warrants -- blank checks for surveillance."
In 2005, a national debate erupted when the Terrorist Surveillance Program authorized by President George W. Bush was revealed in the media. Under that program, begun shortly after the 9/11 attacks, intelligence agencies could spy on communications between U.S. residents and people overseas without a warrant, if it was believed one of the parties was linked to terrorism.
The FISA law passed in 2008 required officials to obtain a warrant to spy on any American -- including, for the first time, Americans who are abroad. The law also established FISA as the exclusive means for authorizing electronic surveillance.
Privacy advocates say if a foreigner overseas is being wiretapped, which does not require a warrant, an American communicating with that person could still have their communications with that foreigner monitored.
But defenders say the law contains provisions to protect privacy and prevent abuse, including a prohibition on spying on Americans except with a warrant.
The National Security Agency, the country's primary surveillance agency, told CNN in a statement: "One of the biggest misconceptions about NSA is that we are unlawfully listening in on, or reading e-mails of, U.S. citizens. This is simply not the case. NSA is unwavering in its respect for U.S. laws and Americans' civil liberties."
Sen. Ron Wyden, D-Oregon, proposed an amendment requiring more disclosure.
"I think we ought to know, generally, how many Americans are being swept up under the legislation," he said during Thursday's debate on the Senate floor.
But defenders of the law say, the effectiveness of espionage programs depends on a level of secrecy.
"It's very hard to have covert and clandestine intelligence-gathering, if it's not secret," said Clifford May, an advocate of national security with the Foundation for Defense of Democracies.
Without reauthorization, the law was scheduled to expire Monday, at the end of 2012.