UPDATE: 11:52 p.m. ET: Sen. Rand Paul (R-Ky.) has yielded back the floor:
Senator and 2016 Republican presidential candidate Rand Paul launched a "filibuster" Wednesday afternoon in protest of the renewal of the Patriot Act, his latest stand against the federal government’s invasive surveillance procedures.
The post-9/11 Patriot Act, which allows for the bulk collection of Americans' phone records by the National Security Agency, is set to expire on June 1 if Congress takes no action. Paul's remarks are technically not a "filibuster" because there is no bill currently being considered.
Paul has promised to sign an executive order to end such government surveillance programs on his first day in office, should he win the presidency.
"There comes to a time in the history of nations when fear and complacency allow power to accumulate and liberty and privacy to suffer. That time is now," Paul said Wednesday. "And I will not let the Patriot Act, the most un-patriotic of acts, go unchallenged."
He later quoted the Fourth Amendment and argued that the text clearly indicates that warrantless surveillance of Americans under the Patriot Act is unconstitutional. He said that the federal government has essentially asserted that all Americans are under "suspicion" by indiscriminately storing their data.
Government surveillance could play prominently in the GOP presidential primary contest, which is heating up just as Congress debates surveillance programs initiated by President George W. Bush's administration and continued under President Barack Obama.
Supporters of the surveillance law, including presidential candidate Sen. Marco Rubio, R-Fla., say it's critical to anti-terrorism efforts. Paul and fellow Sen. Ted Cruz, R-Texas, see the law as a privacy infringement.
Neither Wisconsin Gov. Scott Walker nor former Florida Gov. Jeb Bush has yet to take a formal position on the program, although Bush recently praised the Obama administration's use of big metadata programs that began under Bush's older brother, former President George W. Bush.
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The Associated Press contributed to this report.