In his missile defense speech yesterday, President Bush proclaimed that "the sun comes up on a vastly different world." But some say the sun hasn't quite set on the old world yet.

Bush made arguments for pursuing every type of missile defense system, and declared that the Anti-Ballistic Missile Treaty and the idea of mutually assured destruction that kept nuclear peace since World War II need to be replaced with "new concepts of deterrence."

Not everyone who heard the speech was ready to sign up as a nuclear New Ager. Democrats and other critics responded that the new missile defense systems remain unproven and expensive. And they said the old concepts of deterrence are still useful for keeping peace among the major powers, as well as intimidating smaller "rogue states" such as Iraq and North Korea.

Lawmakers of both parties wondered how Bush planned to cut taxes and spend the $60 billion to $100 billion likely to be needed for minimal missile defense without cutting into other defense priorities.

"The Bush administration is unwisely betting our nation's security against nuclear dangers on unproven and costly missile defenses," said John Isaacs, president of the Council for a Livable World, a nonproliferation group. "The president should focus on minimizing the most urgent nuclear dangers by reducing the Russian nuclear arsenal, securing excess nuclear material in the former Soviet Union and convincing North Korea to end its long-range missile program."

Many nuclear strategy experts also questioned Bush's dismissal of the ABM treaty as an anachronism that "enshrines the past," when the United States and Russia each possess thousands of intercontinental ballistic missiles that remain on alert.

"To abandon the ABM with the hope to get that [missile defense] capacity somewhere down the line would damage the security interests of the United States," said Sen. Joseph R. Biden Jr. (Del.), ranking Democrat on the Senate Foreign Relations Committee.

"The whole principle of fly before you buy is one we should adhere to," said Rep. John F. Tierney (D-Mass.), noting system failures in limited testing. "This is placing the political cart before the technical horse."

It also remained unclear whether, as Bush suggested, the United States can escape the idea that security be based on the "grim premise that we can destroy those who seek to destroy us." If a missile defense system worked completely, it could spark a bigger arms race, some observers noted. If it didn't work completely, then the possibility of mass destruction would still linger.

"If you can't shoot down 100 percent of them [missiles], you haven't gotten rid of mutually assured destruction," said Sen. John F. Kerry (D-Mass.). "And if you can, you set off an arms race to develop a capacity that can't be touched by a missile defense system."

On Monday, a senior administration official said the proliferation of nuclear and long-range missile technology had changed the priority from stopping a Russian attack to stopping a rogue state or leader. The official said that during the Cold War, the specter of mutual destruction made it unlikely that a Soviet leader would initiate a nuclear war. Today, however, with such leaders as Iraq's Saddam Hussein and North Korea's Kim Jong Il, the official said, one could not be so sure.

But others say deterrence works even with these leaders, regardless of what Bush called their "hatred" of America. Noting that veiled threats of nuclear retaliation against Iraq during the Persian Gulf war had kept Saddam Hussein from using chemical weapons, Biden said, "This premise that one day Kim Jong Il or someone will wake up one morning and say, 'Aha, San Francisco' is specious."

Bush tried to make an overture to Russia by urging Moscow to view its relationship with the United States as being far different than it was in the Cold War era, during which nuclear strategy was developed. He said: "Russia is not our enemy, but a country in transition with an opportunity to emerge as a great nation, democratic, at peace with itself and its neighbors."

Some Democrats expressed surprise at Bush's description of a new relationship with Russia. "My first reaction is that this is an unusually friendly description of a Russia that during the campaign was treated in an unusually chilly fashion," said Leon Fuerth, who served as Vice President Al Gore's national security adviser. "It takes a little thought to say we don't need deterrence with Russia."

For many Democrats, the problem with Bush's missile defense speech was a matter of degree and approach. Most Democrats at least support continued missile defense research, and many support the deployment of systems that could be targeted at small countries without unraveling the larger framework of deterrence that governs relations with Russia and China.

"No one is questioning that some form of missile defense is required," said Rep. Neil Abercrombie (D-Hawaii). But he said that some of the more ambitious systems under consideration were "more appropriate to DreamWorks and Steven Spielberg than to actual implementation."

President Bush descends toward the podium at National Defense University at Fort McNair, where he called for "new concepts" of nuclear deterrence.