Cheney launched the ferocious salvo as he accepted the Republican nomination for a second term as right-hand man to his boss, President George W. Bush, at the party convention in New York.

The vice-president, a consummate behind-the-scenes power broker in Washington, launched into Bush's opponent in the November 2 poll for constantly vacillating over issues crucial to US national security.

"His back-and-forth reflects a habit of indecision and sends a message of confusion," Cheney said in an electoral warcry filled the tough "red meat" language Republicans had expected from him.

He lambasted the Democrat for changing his mind on military and national security issues during his 20-year US Senate career.

Kerry's fickleness put America at risk as it faced an unrivalled threat from terrorists armed with chemical, biological or even nuclear weapons and whose "hatred of us is limitless," he said.

To a rolling chorus of cheers, Cheney, 63, warned voters that defending the United States against terrorists was "the greatest challenge of our time" and the outcome of the election would affect US destiny.

"Moments come along in history when leaders must make fundamental decisions about how to confront a long-term challenge abroad and how best to keep the American people secure," Cheney said.

"This nation has reached another of those defining moments," he said.

He suggested that Bush had taken the steps needed to protect the United States, while Kerry was not up to the job.

"History has shown that a strong and purposeful America is vital to preserving freedom and keeping us safe, yet time and again Senator Kerry has made the wrong call on national security," he said.

"On the question of America's role in the world, the differences between Senator Kerry and President Bush are the sharpest, and the stakes for the country are the highest."

The vice president, architect of the Bush doctrine of pre-emptive attacks, also took aim at Kerry's opposition to the war in Iraq, which he said had been a "gathering threat" to the United States.

"On Iraq, Senator Kerry has disagreed with many of his fellow Democrats. But Senator Kerry's liveliest disagreement is with himself.

"Senator Kerry says he sees two Americas," Cheney said, picking up on a Kerry campaign slogan. "It makes the whole thing mutual -- America sees two John Kerrys."

Kerry could get away with changing his mind as a senator, but could not do so as president, he said.