"We could not accept the grave danger of Saddam Hussein and his allies turning weapons of mass destruction against us or our friends and allies," Cheney told the conservative Heritage Foundation on Friday.
Cheney struck back at criticism of the Iraq war that has built over the months since Mr. Bush declared major combat over on May 1. His speech picked up where President Bush left off a day earlier, when the president told listeners in Portsmouth, N.H., "The challenges we face today cannot be met with timid actions or bitter words."
Yet Cheney offered no new evidence that Saddam posed an imminent threat as the administration claimed before the war. The vice president's 25-minute speech also largely dismissed the continuing violence in Iraq, the lack of broad international collaboration, and the failure so far to find any weapons of mass destruction, mentioning only in passing the "difficulties we knew would occur."
The vice president said, "The ultimate nightmare could bring devastation to our country on as scale we have never experienced."
"Instead of losing thousands of lives, we might lose tens of thousands or even hundreds of thousands in a single day of war," Cheney said.
"Remember what we saw on the morning of 9-11. And knowing the nature of these enemies, we have as clear a responsibility as could ever fall to government," Cheney said. "We must do everything in our power to keep terrorists from ever acquiring weapons of mass destruction."
Cheney did not offer new evidence that there was any link between Saddam Hussein and the Sept. 11 attacks. But he cast the Iraq invasion as part of the war on terror. He contrasted the Bush administration's efforts to combat terrorism with what he called previous presidents' "ad hoc" attempts.
"President Bush declined the course of inaction, and the results are there to see," Cheney said.
"It would be reckless in the extreme to rule out action and save our worries until the day they strike," he said. "If the threat from terrorists and terrorist states are permitted to fully emerge, all actions, all words and all recriminations would come too late."
"That is the debate, that is the choice set before the American people and as long as George W. Bush is president of the United States, this country will not permit gathering threats to become certain tragedies," Cheney said.
His speech was the latest in a wave of public appearances and interviews top White House officials are using to answer critics of Mr. Bush's handling of Iraq. The president planned to conduct a half-dozen TV interviews on Monday.
Congressional Democrats labeled the administration's public relations offensive on Iraq "offensive."
House Democratic leader Nancy Pelosi on Thursday accused the administration of seeking a blank check for post-war needs, reports CBS News Correspondent Peter Maer.
"We keep committing more to the reconstruction of Iraq without recognizing the unmet needs we have in our own country," said Pelosi, D-Calif.
Democrats want to cancel some tax cuts for the wealthy to finance post-war Iraq reconstruction. It's part of a counter-proposal to the administration's $87-billion package.
Thursday, six months after the statue of Iraqi leader Saddam Hussein fell in Baghdad, Mr. Bush said he was concerned that "perceptions" didn't reflect the reality of progress in Iraq. He spoke on a day when more than a dozen people, including a Spanish attache and two American soldiers, died in a fresh burst of violence in Baghdad.
"They're trying desperately to undermine Iraq's progress and throw that country into chaos," Mr. Bush said at an Air National Guard base in Portsmouth, N.H. "They believe that America will run from a challenge. They're mistaken. Americans are not the running kind."
Americans, he said, "did not run from Germany and Japan following World War II."
"We helped those countries become strong and decent and democratic societies that no longer waged war on America, and that's our mission in Iraq right now," Mr. Bush said.