Jim Cole, Associated Press
Republican presidential candidate, Sen. Rand Paul, R-Ky. is questioned after a town hall meeting at the Loins Club hall with area residents, Monday, May 11, 2015, in Londonderry, N.H.

PHILADELPHIA — Kentucky Sen. Rand Paul vowed on Monday to "everything possible" to block renewal of the terrorism-era Patriot Act, but the Republican presidential hopeful conceded it may not be enough.

Speaking in front of Philadelphia's Independence Hall, Paul lashed out at the National Security Agency's bulk collection of American citizens' phone records, which many in his own party say are needed to prevent terrorism.

"We will do everything possible — including filibustering the Patriot Act to stop them," Paul said, acknowledging that a filibuster likely wouldn't be enough to block the program. "They have the votes inside the Beltway. But we have the votes outside the Beltway, and we'll have that fight."

The surveillance program will expire on June 1 unless Congress acts.

The federal government's surveillance programs 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.

Congress is debating the reauthorization of the USA Patriot Act, which is set to expire on June 1. 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.

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.