Even if the law expires, the government can get an order from the secret Foreign Intelligence Surveillance Court to compel their cooperation. That court was created 30 years ago for just such a purpose. But McConnell rejects that option. He says the process of getting a court order ties intelligence agents up in red tape.

The 1978 Foreign Intelligence Surveillance Act requires court permission to tap wires inside the United States. Changes in technology since then mean much of the world's computer and phone traffic passes through the United States, much of it on fiber-optic cable. Successive court cases say court orders are needed to listen in on any of them, McConnell said.

To get a court order, intelligence agents have to prove they have "probable cause" to believe a target is foreign agent or terrorist before being allowed to tap a line inside the United States, even if the communication originates and ends in a foreign country.

It is difficult for intelligence agents piecing together shreds of information to get enough to merit probable cause, he said. By the time they can amass enough information to do that, the phone number they wanted to track might already be obsolete, McConnell said.

"More than likely we would miss the very information we need to prevent some horrendous act from taking place in the United States," he said.

The FISA law does make provisions for fleeting targets when there is not time to fill out the paperwork. Within a few days, though, the paperwork must be completed and probable cause proved to get an order approved.

The easy solution, say Democratic congressional leaders, is to extend the current law long enough to allow the House and Senate to work out the differences in their respective surveillance bills. The House finished its version in October, but the Senate did not finish until this week, pushing Congress hard up against the deadline.

The law had been set to expire on Feb. 1. The White House reluctantly agreed to a 15-day extension but refuses to approve any more, and has appealed to House leaders to simply approve the version approved by the Senate, which includes the legal immunity for telecom companies the president wants.

The immunity provision protects phone companies that helped the government in its warrantless wiretapping program conducted outside the authority of the FISA court, a feature the House intentionally left out.

Unable to muster the votes to extend the current law, House leaders say they'd rather let it lapse and operate under the old FISA rules than be pressured by the White House into accepting the Senate bill. House Republicans protested with a walkout Thursday.