Republican presidential contenders Marco Rubio and Rand Paul this week butted heads over the NSA telephone metadata program, with the Florida senator backing an extension of the bulk-collection effort and his libertarian Kentucky colleague threatening a filibuster over the Patriot Act's possible reauthorization.

Sen. Rubio insisted in a USA Today op-ed that "the government is not listening to your phone calls or recording them unless you are a terrorist" or have contact with terrorists abroad.

"Today our nation faces a greater threat of terrorist attack than any time since Sept. 11, 2001," he warned.

The presidential hopeful also made reference to a recent decision by the Second Circuit Court of Appeals, which had ruled that the Patriot Act -- first passed in the immediate aftermath of the Sept. 11 terrorist attacks on New York and Washington -- does not authorize the National Security Agency to gather up millions of phone records on an ongoing basis.

But "despite recent court rulings, this program has not been found unconstitutional, and the courts have not ordered a halt to the program," Sen. Rubio insisted. "There is not a single documented case of abuse of this program."

Privacy advocates have long attacked the metadata surveillance program, and in the wake of NSA leaker Edward Snowden's revelations, even the White House said it was looking into alternatives to the current system of collecting Americans' phone records.

Echoing critics' concerns, meanwhile, Sen. Paul promised to oppose a reauthorization of a number of provisions within the Patriot Act, which are due to expire in the coming weeks, the Hill noted.

"I'm going to lead the charge in the next couple of weeks as the Patriot Act comes forward," the senator told the New Hampshire Union Leader. "We will be filibustering. We will be trying to stop it. We are not going to let them run over us. And we are going to demand amendments and we are going to make sure the American people know that some of us at least are opposed to unlawful searches."