The U.S. Senate resumed debate Tuesday on legislation that would extend a controversial U.S. National Security Agency surveillance program targeting suspected terrorists, even as members of President George Bush's administration threatened a veto of the bill if senators make changes to it.

A group of Democratic senators pushed for an amendment to the bill that would take out the so-called lawsuit immunity provisions for telecommunication carriers that allegedly participated in the NSA program. A second amendment being considered would delay a court decision on whether the more than 40 lawsuits against telecom carriers should move forward for a year, until inspector generals from several U.S. agencies could issue reports on the legality of the surveillance program.

On Monday. U.S. Attorney General Michael Mukasey and Director of National Intelligence Mike McConnell sent a letter to Senate leaders saying a delay in a decision on telecom immunity would be "unacceptable."

"Providing prompt liability protection is critical to national security," they wrote. "We, as well as the president's other senior advisers, will recommend that the president veto any bill that includes such an amendment."

Critics suggested the Bush administration's threat of a veto means the president is placing a greater priority on telecom immunity than on continuing a surveillance program Bush has called essential for fighting terrorism.

In addition, the telecom immunity provisions would violate the separation of powers set up in the U.S. Constitution, said Senator Sheldon Whitehouse, a Rhode Island Democrat. The telecom immunity provisions are the "most extreme example of congressional interference I can find" on the authority of the U.S. court system, Whitehouse said.

Whitehouse suggested the telecom immunity provisions would likely be challenged before the U.S. Supreme Court.

Senate Majority Leader Harry Reid, a Nevada Democrat, said a vote on the bill, the Foreign Intelligence Surveillance Act (FISA) Amendments Act, could happen as soon as Wednesday.

Civil liberties organizations and many Democrats have objected to the NSA program because it was done in secret and allowed surveillance of U.S. residents who were talking to overseas suspects without court oversight such as warrants. The program was illegal under the U.S. Constitution's Fourth Amendment prohibiting unreasonable search and seizure, critics have said.

The FISA Amendments Act is part of a compromise between some congressional Democrats and the administration. It would allow the NSA program to go forward with some court oversight, and it would send the dozens of outstanding lawsuits against telecom carriers for their alleged participation to a district court, which will review whether they should be dismissed.