The charge brought a counter-attack from his Democratic White House rival, Senator John kerry, who accused Bush of favoring missile defense over other measures to blunt the threat posed by terrorists.

The president made his remarks during an election-year visit to a Boeing military equipment factory near here, noting that Boeing engineers had placed "the first ballistic missile interceptor into its silo" at a base in Alaska.

Calling the controversial project "necessary to protect us against the threats of the 21st century," he noted that the late president Ronald Reagan, also a Republican, had sowed the seeds of the system.

"Those who oppose this ballistic missile system really don't understand the threats of the 21st century. They're living in the past. We're living in the future. We're going to do what's necessary to protect this country."

Bush, whose aides say they are concerned about missile threats from North Korea and Iran or from terrorists who acquire such technology, did not name any particular country or group as a potential attacker.

"We want to continue to perfect this system, so we say to those tyrants who believe they can blackmail America and the free world: 'You fire, we're going to shoot it down'," he said.

Bush, who hopes to win a second four-year term in the November 2 elections, also did not mention Kerry by name.

But in a sharply worded response, Kerry national security adviser Rand Beers derided Bush's "near obsession with missile defense" and said that "the greatest threat facing our homeland comes from terrorists."

In the months before the September 11, 2001 attacks, "Bush and his closest advisors were preoccupied with missile defense and their misunderstanding about the threats we face continues to this day," Beers said in a statement.

Kerry "understands the importance of facing our most pressing national security threats while continuing to develop and deploy a national missile defense which we know will work," added Beers.

The senator from Massachusetts pushed in 1994 for a series of military spending cuts, including reductions in the national missile defense program, but the Senator never voted on his proposal.

Before the September 11 attacks, missile defense was Bush's signature national security issue, and he has continued to argue that even an imperfect system could convince nations not to develop such an arms program.

The Pentagon plans to deploy some 10 missile interceptors in Alaska and California this year. Bush has approved 10 billion dollars for the project in the 2005 defense budget, which totals 400 billion dollars.

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