WASHINGTON — President Bush threatened a veto yesterday in the debate to update terrorist surveillance laws, assailing Democratic plans to deny protection from lawsuits for telecommunications providers that let the government spy on American residents after the September 11, 2001, attacks. The threat came in a 12-page letter to Senate leaders from Attorney General Michael Mukasey and National Intelligence Director Mike McConnell. It was issued as lawmakers prepare to vote on legislation seeking to update a 1978 surveillance law without violating privacy rights.

“If the president is sent a bill that does not provide the U.S. intelligence agencies the tools they need to protect the nation, the president will veto the bill,” Mr. Mukasey and Mr. McConnell wrote. The letter was sent to Senate leaders and the top Democrats and Republicans on the Senate Judiciary and Intelligence committees.

Senate Majority Leader Harry Reid, a Democrat of Nevada, said the letter was premature since there still isn’t any legislation yet. The existing surveillance law will expire February 15. Mr. Bush has said he would resist extending it again.

After nearly two months of legislative wrangling, Mr. Reid announced the Senate would begin voting on amendments today. Debate began yesterday evening.

The administration’s veto threat was aimed at amendments that would bar retroactive immunity to phone companies and other telecom providers that have given the government access to e-mails and phone calls linked to people in America.

“Private citizens who respond in good faith to a request for assistance by public officials should not beheld liable for their actions,” Mr. Mukasey and Mr. McConnell wrote.