The Justice Department warned Wednesday that the National Security Agency will have to start winding down its controversial bulk collection of Americans' phone records by the end of the week if Congress does not renew the Patriot Act. 

The warning was contained in a memo to lawmakers outlining the implications of a deepening dispute on Capitol Hill over the counterterror law. The department said several tools used by U.S. intelligence officials to combat terrorism will lapse at the end of the month without congressional action. And even before that, the DOJ warned, the NSA would have to start suspending its phone data dragnet to prepare. 

While civil liberties advocates have long opposed these programs anyway, the prospect of intelligence analysts having to effectively suspend work has infuriated hawkish lawmakers. House Homeland Security Committee Chairman Michael McCaul, R-Texas, said he hopes lawmakers act soon "for the safety of the country." 

Lawmakers are currently grappling with dueling surveillance bills. One would effectively end, and replace, the controversial NSA collection program. Another option is to simply reauthorize the post-Sept. 11 Patriot Act. But the issue has divided Republicans and Democrats, cutting across party lines and scrambling efforts to figure out a way forward. 

With time already running short, that debate got sidelined further Wednesday as Republican Sen. Rand Paul took to the Senate floor to rail against the Patriot Act for nearly 11 hours, beginning at 1:18 p.m. EDT and not ending until 11:49 p.m. EDT. 

"I will not let the Patriot Act, the most un-Patriotic of acts, go unchallenged," Paul said at the start. Paul, a candidate for the Republican nomination for president, later added from the floor, “The people don’t want the bulk collection of their records, and if we were listening, we would hear that.” 

Congress must deal with the law's fate before lawmakers leave town for the weeklong Memorial Day recess, or it will lapse. Faced with the expiration of the law on June 1 and uncertainty on Capitol Hill, the department circulated a memo on Wednesday that described the powers that could soon expire. 

"After May 22, 2015, the National Security Agency will need to begin taking steps to wind down the bulk telephone metadata program in anticipation of a possible sunset in order to ensure that it does not engage in any unauthorized collection or use of the metadata," the Justice Department said.

Last week, the House backed the USA Freedom Act, which would replace bulk collection with a system to search the data held by telephone companies on a case-by-case basis. The vote was 338-88, and House Republican and Democratic leaders have insisted on their bill.

But Senate Majority Leader Mitch McConnell, R-Ky., and several other top Republicans prefer to simply reauthorize the post-Sept. 11 law. McConnell has agreed to allow a vote on the House bill, but has indicated that there may not be enough votes to pass it in the Senate. 

There's also the possibility of a short-term bill, though it's unclear whether it could muster enough support to pass. 

If Congress fails to act, several key provisions of the law would expire, including the bulk collection; a provision allowing so-called roving wiretaps, which the FBI uses for criminals who frequently switch cell phones; and a third that makes it easier to obtain a warrant to target a "lone wolf" terror suspect who has no provable links to a terrorist organization.

The Justice Department said if Congress allows the law to expire and then passes legislation to reauthorize it when lawmakers return to Washington the week of June 1 it would "be effective in making the authorities operative again, but may expose the government to some litigation risk in the event of legal challenge."

The White House backs the House bill and has pressed for the Senate to approve the legislation and send it to President Obama for his signature.

The House bill is the result of outrage among Republicans and Democrats after former NSA contractor Edward Snowden's revelations about the NSA program.

Prior to his speech, Paul sent out a fundraising appeal describing his effort as a "filibuster" to stop the extension of the "unconstitutional and illegal domestic spying programs."

"I will not rest. I will not back down. I will not yield one inch in this fight so long as my legs can stand," he wrote.

Although Paul called it a filibuster, it technically fell short of that definition since the Senate was not officially considering the Patriot Act when he started talking. 

Fox News' Chad Pergram and The Associated Press contributed to this report.