The Bush administration intensified its defense of the anti-terrorism Patriot Act (search) on Thursday, threatening to veto legislation in Congress that would scale back key provisions.

Attorney General John Ashcroft (search), in a letter to Senate leaders, said the changes proposed in the Security and Freedom Ensured Act (search), known as SAFE, would "undermine our ongoing campaign to detect and prevent catastrophic attacks."

Ashcroft told reporters that President Bush would veto the bill if it reached his desk.

The threat came a week after Bush, in his State of the Union address, urged Congress to reauthorize the Patriot Act before it expires in 2005. A few months earlier, Ashcroft embarked on a 32-city speaking tour in a bid to answer critics who contend the law threatens civil liberties and privacy rights.

Ashcroft said the political offensive "reflects the stakes America has in the war on terror. When American lives are at stake, we need to have all the capacities to disrupt and to defeat terrorism that we've been successfully using over the last 28 months."

The Patriot Act, passed shortly after the Sept. 11, 2001, terror attacks, expanded the government's wiretap and other surveillance authority, removed barriers between FBI and CIA information-sharing, and provided more tools for terror finance investigations.

Anthony Romero, executive director of the American Civil Liberties Union and a staunch critic of the new law, said the veto threat shows that the Bush administration is on the defensive. The ACLU has filed a lawsuit in federal court challenging a key portion of the law, and 241 state and local governments also have gone on record opposing it.

"The attorney general's attack on the SAFE Act shows how out of step the Bush administration is with growing national concern over the Patriot Act," Romero said.

Earlier this month in Los Angeles, a federal judge issued the first court ruling striking down a portion of the law. U.S. District Judge Audrey Collins ruled that provisions barring "expert advice or assistance" to groups designated foreign terrorist organizations was too vague, threatening First and Fifth Amendment rights.

The SAFE Act, which has not yet had a hearing in either the House or Senate, was introduced last fall by Sens. Larry Craig, R-Idaho; Dick Durbin, D-Ill.; and other lawmakers of both parties who say the Patriot Act has gone too far.

"Attorney General Ashcroft's response today is an unfortunate overreaction to a reasoned and measured effort to mend the Patriot Act," Durbin said Thursday. "I believe it is possible to combat terrorism and preserve our individual freedoms at the same time.

"This legislation restores the necessary checks and balances to the system."

The bill would modify so-called "sneak and peek" search warrants that allow for indefinitely delayed notification when a person's property is searched, mandating such notice within a week's time.

In addition, warrants for roving wiretaps used to monitor a suspect's multiple cell phones would have to make sure the target was positively identified and was present at the site being monitored before information could be collected.

The legislation also would reinstate standards in place prior to passage of the Patriot Act regarding library and other business records by forcing the FBI to show it had reason to believe the person involved was a suspected terrorist or spy. The measure would impose expiration dates on nationwide search warrants and other Patriot Act terms, providing for congressional review.