WASHINGTON -- The abrupt decision by Senate leadership on Thursday to reconsider their opposition to a controversial Patriot Act amendment came as a bit of a surprise to Congress followers.

But a likely explanation for the reversal began to emerge hours later.

With the national security law set to expire at midnight on Thursday, the Obama administration has applied intense pressure to congressional lawmakers to finalize an extension without any lapse. The pressure, apparently, has caused Senate leadership to bend to lawmakers critical of the bill and streamline the procedures needed to get the law passed.

On Wednesday, James Clapper, the Director of National Intelligence, wrote a letter to Senate Majority Leader Harry Reid (D-Nev.) and Minority Leader Mitch McConnell (R-Ky.) warning about potential national security hiccups should the Patriot Act not be extended before its expiration. In his message, Clapper detailed the operational procedures that would be disadvantaged as well as to note the heightened threat that exists following Osama bin Laden's death.

"At this time of enhanced threats to the United States, the intelligence and law enforcement communities are working at a rapid pace to analyze and exploit recently collected intelligence to safeguard our national security," he wrote. "The information obtained at the Osama Bin Laden compound must be quickly analyzed for any indications and warning of terrorist plots and attack plans. As part of this effort we are using all our collection authorities to investigate and prevent terrorist attacks."

That letter was the second one Clapper had sent to Congress this week. On Monday, he and Attorney General Eric Holder wrote lawmakers to strongly urge that expiring provisions be passed without delay, including a section that provides authority for roving surveillance of so-called "lone wolf" terrorists (non-U.S. persons who engage in international terrorism but may not be part of an identified group) and a section that provides expanded authority for the FBI to obtain business records that may contain terrorist leads.

All of this, the administration stresses, is being done under legal authority under the Foreign Intelligence Surveillance Act and constitutes "critical" tools for national security operations.

Senators from both parties have scoffed at the logic, arguing that the White House is essentially asking lawmakers to forfeit civil liberty concerns in the face of a nebulous terrorist threat. A good chunk of the chamber will filibuster the Patriot Act's extension when it ultimately comes to a vote.

Sen. Rand Paul (R-Ky.) will be among them, though in addition to his philosophical concerns, he has also tried to stall the bill in hopes of securing a vote on an amendment that would prohibit law enforcement from using gun purchase records to track terrorist suspects. On Wednesday evening, leadership seemed content to stand firm against allowing a vote on that amendment, even if it meant letting the Patriot Act lapse for a brief period of time. On Thursday morning, it appeared to have acquiesced.

A senior administration official, underscoring the pressure being applied to Congress, emailed the following to HuffPost:

The FISA provisions that are set to expire are critical tools that have been used in numerous sensitive and significant operations to detect and disrupt threats to the United States. Failure to pass this legislation with sufficient time for the President to sign it before it expires - at midnight on Thursday - poses a significant risk to U.S. national security. The bottom line is that if these provisions are allowed to lapse, even temporarily, the nation will be less safe. Today, there are significant, ongoing terrorism and counterintelligence investigations where these tools are or may prove to be necessary. We cannot risk any lapse in these critical authorities, regardless of how brief that is. Particularly now, at a time of enhanced threats to the U.S., the intelligence and law enforcement communities are working at a rapid pace to analyze and exploit recently collected intelligence to safeguard our national security.