President Bush issued a veto threat Tuesday in the debate to update terrorist surveillance laws, rebuking Democratic plans to deny retroactive legal protections for telecommunications providers that let the government spy on U.S. residents after the Sept. 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," wrote Mukasey and McConnell.

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, D-Nev., said the letter was premature since there still isn't any legislation yet.

"It's a little early to have a veto threat," he said.

The Senate has attempted for two months to debate the new surveillance bill but has hit repeated procedural snags.

Reid blamed Bush, Vice President Dick Cheney and Senate Republicans for the delay. He accused them of trying to stall until the existing surveillance law expires. Then the Senate will be under pressure to adopt a surveillance bill the administration favors and "stuff it down the House's throat."

The existing surveillance law will expire Feb. 15. Bush has said he would resist extending it again.

However, Senate Republican leader Mitch McConnell of Kentucky on Tuesday predicted the Senate would complete the bill this week.

The administration's veto threat was aimed at legislative 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 the United States. Without the retroactive protections, the letter noted, telecom providers might be unwilling to help the government track down terror suspects in the future as they were asked to do in the days following the 2001 attacks.

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

A bill already approved by the Senate Intelligence Committee "is not perfect," Mukasey and McConnell wrote. But since it provides the legal shields, they said they would support it if the amendments are dropped.

The Senate could vote on the surveillance bill and amendments this week.

Some 40 civil lawsuits have been filed against telecommunications companies. They carry with them a threat of crippling financial penalties, which the White House says could bankrupt the companies.

Congress has struggled to strike a balance between catching terrorists and improperly spying on U.S. residents since last summer, when it sought to update the 1978 Foreign Intelligence Surveillance Act. That law requires the government to get approval from a secret court when it seeks to electronically eavesdrop on suspected terrorists or spies in the United States. The law does not apply to government wiretaps on people outside the country.

Over the years, however, foreign communications have been routed through technology based in the United States — raising the question of whether the government should have FISA court approval to listen in on those conversations. The bill being debated now seeks to resolve that issue.

Civil rights and privacy advocates say current law, which Congress approved in August in a hasty attempt to update the 1978 version, still allows the government to eavesdrop on Americans without court oversight. That law was set to expire Feb. 1 but was extended for two weeks as Congress works to hammer out a compromise.

The administration also rejected proposals to have the secret FISA court decide whether to give immunity to telecom firms, arguing that doing so could merely result in lengthy legal battles.

"It is for Congress, not the courts, to make the public policy decision whether to grant liability protection to telecommunication companies who are being sued simply because they are alleged to have assisted the government in the aftermath of the September 11th attacks," Mukasey and McConnell wrote.

Similarly, the administration said it would oppose efforts to let the new law expire after four years instead of the six as proposed in the Intelligence Committee bill.

Not all of amendments being debated drew the administration's ire. Mukasey and McConnell said they do favor two other proposed plans to expand FISA's authority: to eavesdrop on people suspected of proliferating weapons of mass destruction and to quickly resolve challenges to the government's collection of intelligence.