Sen. Rand PaulRandal (Rand) Howard PaulHouse bill set to reignite debate on warrantless surveillance Authorizing military force is necessary, but insufficient GOP feuds with outside group over analysis of tax framework MORE (R-Ky.) on Wednesday called the National Security Agency's (NSA) collection of Verizon customers’ phone records an "astounding assault on the Constitution."[ Watch ]

"After revelations that the Internal Revenue Service targeted political dissidents and the Department of Justice seized reporters’ phone records, it would appear that this Administration has now sunk to a new low," he said in a statement.

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"If the President and Congress would obey the Fourth Amendment we all swore to uphold, this new shocking revelation that the government is now spying on citizens’ phone data en masse would never have happened," Paul added.

The Guardian newspaper on Wednesday published a copy of a secret court order compelling Verizon to take part in a massive data mining program to help spot suspected terrorists.

The NSA obtained information on phone numbers, the length and location of calls but no data on the content of conversations or messages. The order covered all Verizon calls since April 25, including millions of people not suspected of being involved in terrorism.

The administration on Thursday defended the NSA’s practices, saying it was “critical” to national security efforts.

An administration official said the program helped "discover whether known or suspected terrorists have been in contact with other persons who may be engaged in terrorist activities, particularly people located inside the United States."

The leaders of the Senate Intelligence Committee said they had been informed of the phone sweeping and that it had been going on since 2007.

“Everyone’s been aware of it for years, every member of the Senate,” said Sen. Saxby ChamblissSaxby ChamblissLobbying World Former GOP senator: Let Dems engage on healthcare bill OPINION: Left-wing politics will be the demise of the Democratic Party MORE (R-Ga.), the ranking Republican on the panel.

Chambliss and Chairwoman Dianne FeinsteinDianne Emiel FeinsteinGun proposal picks up GOP support Gingrich: Banning rapid fire gun modification is ‘common sense’ House bill set to reignite debate on warrantless surveillance MORE (D-Calif.) also defended the program.

“There have been approximately 100 plots and also arrests made since 2009 by the FBI,” Feinstein said. “Terrorists will come after us if they can, and the only thing we have to deter this is good intelligence.”

Many lawmakers though said they were not aware of the full extent of the NSA’s practices and called on the administration to justify its legal rationale.

Democratic Sen. Jeff MerkleyJeffrey (Jeff) Alan MerkleyOvernight Energy: Dems take on Trump's chemical safety pick Dem senator slams Trump for dedicating golf trophy to hurricane victims Dem senator compares Trump to Marie Antoinette MORE (D) called it an "outrageous breach of Americans' privacy."

Sen. Dean HellerDean Arthur HellerNevada senators urge airlines to enact new policies after Las Vegas shooting Dems plan to make gun control an issue in Nevada GOP establishment doubts Bannon’s primary powers MORE (R-Nev.), who along with Merkley and Sen. Mike LeeMichael (Mike) Shumway LeeWhatever you think the Alabama special election means, you’re probably wrong How the effort to replace ObamaCare failed Overnight Regulation: Trump temporarily lifts Jones Act for Puerto Rico | Bill would exempt some banks from Dodd-Frank | Senators unveil driverless car bill MORE (R-Utah) backed an amendment requiring the U.S. attorney general to disclose Foreign Intelligence Surveillance Court (FISA) decisions, also criticized the NSA's actions.

"This is yet another example of government overreach that forces the question, ‘What sort of state are we living in?’ There is clearly a glaring difference between what the government is doing and what the American people think they are doing," Heller said.