WASHINGTON -- Sen. Rand Paul (R-Ky.) introduced legislation Friday that would prevent the government from obtaining the phone records of Americans without "a warrant based on probable cause," following reports that the National Security Agency has secretly been collecting the records of millions of Americans.

The Guardian reported Wednesday that the NSA is collecting call data from millions of Verizon customers under a top-secret U.S. court order. The news, which comes on the heels of reports that the Justice Department had been investigating leaks involving the Associated Press and Fox News , prompted further concerns about government overreach.

It appeared to be the moment Paul, a vocal defender of civil liberties, was waiting for. He announced his bill, "The Fourth Amendment Restoration Act of 2013," in a statement Thursday, as his colleagues on Capitol Hill were busy defending the NSA program .

"The revelation that the NSA has secretly seized the call records of millions of Americans, without probable cause, represents an outrageous abuse of power and a violation of the Fourth Amendment to the Constitution," Paul said. "The bill restores our Constitutional rights and declares that the Fourth Amendment shall not be construed to allow any agency of the United States government to search the phone records of Americans without a warrant based on probable cause."

Paul strongly condemned the surveillance, calling it an "astounding assault on the Constitution." He accused President Barack Obama of being more "bent towards authoritarianism" than former President George W. Bush.

Paul announced his measure before the Washington Post reported Thursday evening about a government program called PRISM , which tracks information from nine leading U.S. Internet companies. But he also introduced a bill last month that would extend Fourth Amendment guarantees to electronic communications. The bill would require judges to grant specific warrants to the government in order for it to obtain information on Americans' use of email, the Internet, credit cards, cell phones, and other forms of modern communication.

Obama said Friday he "welcomes" a debate on the issue of national security and privacy rights, but it remains unlikely that Paul's attempt to revisit the issue legislatively will gain much traction. Lawmakers acknowledged Thursday that Congress approved the NSA program and was regularly briefed on it. Members of the Senate Intelligence Committee forcefully defended the steps as necessary to protect Americans from the threat of terrorism.

And although Paul has garnered national attention over his pledge to protect civil liberties, most recently through a 13-hour talking filibuster on the use of drone strikes, his own record is rather mixed. The senator has supported the surveillance of foreign exchange students from the Middle East and signaled support for the imprisonment of individuals who attend "radical" speeches. He has consistently opposed the Patriot Act, but opposes shutting down the prison camp at Guantanamo Bay.

Paul also received $4,800 from PayPal co-founder Peter Thiel for his 2010 Senate run, the maximum donation allowed that year, according to Federal Election Commission data. Thiel is also the founder of Palantir Technologies, a controversial defense contractor that profits from providing spy services to the CIA, FBI and other agencies. Paul's association with Thiel is nonetheless modest compared to that of his father, former Rep. Ron Paul (R-Texas), who during his 2012 presidential run received his largest donations from Thiel.

Paul Blumenthal contributed reporting.

Read the full text of the bill: