PHILADELPHIA – Sen. Rand Paul (R-Ky.) threw down a gauntlet on domestic spying and civil liberties concerns at Independence Hall on Monday, highlighting the stark divide between the libertarian and hawk wings of the 2016 Republican field.

“Our founding fathers would be appalled to know we are writing one single warrant and collecting everyone’s phone records all of the time,” Paul declared as he stood with supporters in front of the historic site.

The difference between Paul and several of his rivals on the issue, rhetorically and substantively, couldn’t have been starker. The same day Paul warned of creeping encroachments on liberty just steps from where the Constitution was debated and adopted, Sen. Marco Rubio (R-Fla.) unveiled a new slogan, “Nothing matters if we aren’t safe,” on his website, New Jersey governor Chris Christie delivered a speech calling concerns about spying “ridiculous,” and Paul’s national security nemesis Sen. Lindsey Graham (R-S.C.) teased a presidential announcement on June 1. Paul criticized Graham on Monday for saying this weekend he would respond to requests by suspected terrorists for a lawyer with a lethal drone strike instead.

RELATED: House votes to end NSA surveillance program

Promising a “great and momentous” fight over the National Security Agency’s intelligence programs, Paul pledged to filibuster the PATRIOT Act renewal even as he acknowledged he did not have the votes in the Senate to ultimately defeat it. He brushed aside critics’ claims that curbing the NSA’s domestic surveillance would endanger the homeland.

“One Senator came up to me and he said ‘If you defeat the PATRIOT Act what will happen, how could we possibly survive?’” Paul said. “I said maybe, just maybe, we can rely on the Constitution for a few hours.”

Thanks to everyone who joined me today @INDEPENDENCENHP pic.twitter.com/VO9BstK03r — Dr. Rand Paul (@RandPaul) May 18, 2015

NSA spying has increasingly occupied the GOP field’s attention as the Senate debates reauthorizing phone record surveillance powers under the PATRIOT Act that expire on June 1. A federal court ruled this month that the NSA’s bulk collection of metadata – information about domestic phone calls, but not their content – was unconstitutional absent changes by Congress, a development hailed by Paul and Sen. Ted Cruz (R-Texas), another critic of the program.

Civil liberties issues have long been a calling card for Paul and he’s counting on them now to raise his profile in a Republican field that could include a dozen or more significant rivals. In addition to spying, Paul repeated his call on Monday for a drawdown of the war on drugs, condemned mass incarceration and mandatory sentences, and criticized police seizures of cash and property from suspects who are not charged with a crime.

“We can’t be the party of family values if we’re locking up all these dads, particularly from minority families,” Paul said.

Paul’s positions stem from his broader libertarian philosophy, but they also form the backbone of his political strategy for the primary and general election. He’s hoping his break from traditional GOP positions on crime, civil liberties, and foreign policy will attract new voters – especially young people, minorities and independents.

“I think there are very few Republicans who would give them any reason to switch,” Paul said. “I do think that I’m one Republican who would cause people to switch. At every rally we go to we have Democrats coming up and independents saying they’ll switch.”

As part of that effort, Paul said he would campaign for votes in Philadelphia, whose overwhelmingly liberal lean has powered Democrats to statewide victories in every presidential election since 1988.

On foreign policy, Paul reiterated his view – unique within the GOP field – that the Iraq War not only should not have been waged in hindsight, but that it should never have been waged even with the knowledge available in 2003.

“The unintended consequences of the Iraq War are ISIS, instability, chaos, and we’re more at risk now,” Paul said in a discussion at the National Constitutional Center earlier in the day. He argued others in the GOP were making a similar mistake by calling for the ouster of Syrian dictator Bashar al-Assad, which he warned would invite Islamic radicals to exploit the resulting power vacuum.

“It is exactly the same question [as Iraq]: Should we depose Assad, a strongman, and what will happen if we do?” he said.

Paul challenged Democratic frontrunner Hillary Clinton, who has not given a sit-down interview since announcing her campaign, to address her support for the U.S. bombing campaign that contributed to Muammar Gaddaffi’s ouster in Libya in 2011 amid a civil war. The country has since slid into internal conflict again and Islamic State militants have gained a foothold, committing atrocities that include a mass execution of Egyptian Christians.

“We need to ask her the same question that was asked of Jeb Bush, but ask about Libya,” he said. “Are we better off or worse off with the invasion of Libya?”

Paul broke with some of his libertarian allies in supporting military strikes against ISIS after initially expressing skepticism over such a move, but he has continued to espouse a general philosophy of non-intervention in foreign conflicts. Polls shows Republicans are increasingly concerned with national security even more than they are about the economy, which dominated the 2012 race. Paul has his work cut out for him to sell conservative voters on his more dovish views while his rival candidates offer more bellicose rhetoric and threaten to launch expanded military operations, especially against Iran if nuclear negotiations fall through.