Dick Cheney led a lacerating assault on John Kerry's Oval Office credentials last night, charging that the Democratic challenger's "habit of indecision" left him too misguided and muddled to lead America at a defining moment in history.

The Vice President said that Mr Kerry had been one vote among 100 during his 20 years in the Senate. "But the presidency is an entirely different proposition. A Senator can be wrong for 20 years without consequence to the nation. But a president always casts the deciding vote."

Mr Cheney was preceded to the podium before a highly charged Madison Square Garden audience by Zell Miller, a maverick Democratic Senator from Georgia given a prime-time speaking slot by Republican strategists to deliver his own excoriating verdict on Mr Kerry.

He said the Massachusetts Senator had been "more wrong, more weak and more wobbly than any other national figure".

After two days in which Mr Bush's leadership in the War on Terror and the compassionate face of his administration have been trumpeted, day three of the Republican convention was devoted to subjecting Mr Kerry's political career to merciless criticism.

Mr Cheney said that a strong and purposeful America was vital to preserve freedom. "Yet time and again Senator Kerry has made the wrong call on national security", opposing Ronald Reagan's military build-up and the 1991 Gulf War.

He said that Mr Kerry denounced US military action when other countries did not approve, "as if the whole object of our foreign policy were to please a few persistent critics".

But Mr Bush had made clear that "there is a difference between leading a coalition of many and submitting to the objections of a few".

He said that Mr Kerry was campaigning for the position of commander-in-chief. "Yet he does not seem to understand the first obligation of a commander-in-chief, and that is to support American troops in combat."

On Iraq, Mr Cheney said the Massachusetts Senator had disagreed with many fellow Democrats. "But Senator Kerry's liveliest disagreement is with himself. His back and forth reflects a habit of indecision and sends a message of confusion.

"And it is part of pattern," he said, before reciting Mr Kerry's vote for, and subsequent objections to, Mr Bush's central education and justice reforms.

"Senator Kerry says he sees two Americas. It makes the whole thing mutual. America sees two John Kerrys."

By contrast, Mr Cheney said the re-election of Mr Bush, "this good man" who takes decisions "with the wisdom and humility Americans expect in their president", was critical to seeing through "a war we did not start and have no choice but to win".

Mr Cheney also put his direct opponent, John Edwards, on notice that he would give no concession to the North Carolina Senator's life story. Mr Edwards never makes a speech without referring to his humble roots and the mill in which his father worked.

Mr Cheney trumped him, talking of his grandfather who was a cook on the Union Pacific Railroad and lived in a railroad car.

Mr Cheney has seen his approval ratings fall to the low 40s, but he remains a darling of the Republican grassroots. His criticisms of Mr Kerry sparked chants of "flip-flop" as delegates waved their hands from side to side.

He is, however, a taciturn performer at the best of the times. He was joined on stage after his speech by his wife, one of his daughters and four-grandchildren as Republican image-makers sought to soften his Dr Strangelove image.

But it was a finger-jabbing performance from Mr Miller that drew some of the loudest cheers of the night as well as prompting pantomime boos at mention of Republican bogeymen.

Mr Miller said he had decided to cross over and support Mr Bush's re-election because he had asked himself which candidate would better protect his family, and "because my family is more important than my party".

His speech was a 15-minute diatribe against Mr Kerry mixed with glowing tributes to Mr Bush and the US military.

Mr Miller is not untypical of the breed of conservative Democrats who have seen their grip on power in the South prised away by Republicans over the past generation.

Furious Democrats have branded him as "Zig-Zag Zell" and "Zellout", pointing out that he has a new book out.

But his testimony, albeit occasionally wild-eyed, was beamed around the country in prime time and will not help Mr Kerry in hunting grounds he once had his eye on such as Louisiana and Arkansas.

"Senator Kerry has made it clear that he would use military force only if approved by the United Nations," he said.

"Kerry would let Paris decide when America needs defending. I want Bush to decide.

"John Kerry, who says he doesn't like outsourcing, wants to outsource our national security. That's the most dangerous outsourcing of all. This politician wants to be leader of the free world. Free for how long?"

By contrast, he said Mr Bush wanted to "grab terrorists by the throat and not let them go to get a better grip". But from Mr Kerry "they get a 'yes-no-maybe' bowl of mush that can only encourage our enemies and confuse our friends".

Mr Miller regarded Mr Bush as "a straight talker and a straight shooter". He admired him, "and the fact that he is unashamed of his belief that God is not indifferent to America".

He said: "I have knocked on the door of this man's soul and found someone home, a God-fearing man with a good heart and a spine of tempered steel".

Mr Bush makes his formal acceptance speech tonight, the final day of the convention.

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