John Kerry took a calculated gamble last night with an acceptance speech to the Democratic convention that tore into the presidency of George W Bush, setting the stage for a bruising battle between the two men in the final three months before November's presidential election.

All this week an energised and united Democratic party has fought hard to suppress the visceral anti-Bush sentiment of its activists gathered here to nominate Mr Kerry for president. Eager to project a positive image to the broader American public beyond Boston, speakers stuck mostly to a script that avoided negative attacks on the President. Instead they focused hard on countering popular doubts about their candidate's readiness to lead the country at a time of national peril, emphasising his military record as a decorated Vietnam veteran and presenting a picture to the public of a party that was pro-military and would not shy away from tough decisions in the war on terrorism.

But last night, the party's nominee for President himself , in a surprisingly aggressive speech that at times dripped with contempt for the incumbent President, launched broadside after broadside against Mr Bush.

Mr Kerry's main target was the Iraq war. He accused the President of undermining America's national security and his own war on terrorism by misleading the country into a conflict with Iraq over Saddam Hussein's weapons of mass destruction programmes, by planning inadequately for war, by prematurely declaring victory, and by alienating America's allies through an assertive unilateralism.

"As President I will bring back this nation's time-honoured tradition: the United States of America never goes to war because we want to. We only go to war because we have to," he said.

In attacking the Bush administration's conduct of the Iraq war, Mr Kerry came closer to outright condmenation of it than he has since the fall of Baghdad more than a year ago. In a conscious echo of Mr Bush's own pledge four years ago to restore "honour and integrity to the White House" after the scandal-ridden Clinton years, Mr Kerry said he would return "trust and credibility" to the presidency, in one of a number of references to the presentation of faulty intelligence evidence about Iraqi WMD before the invasion of Iraq.

The Democratic candidate also launched into Mr Bush's economic record, his health care and education policies, and his cabinet, notably Dick Cheney, the Vice President, Donald Rumsfeld, the Defense Secretary, and, to tumultuous cheers, his conservative Attorney General, John Ashcroft, whom he accused of undermining the United States constitution. In a populist note, he invoked the notorious Enron collapse in castigating the Bush administration's tax policy for corporations.

He even questioned the appropriateness of Mr Bush's public professions of his born-again Christianity. "I dont wear my religion on my sleeve ... I don't want to claim that God is on our side."

Mr Kerry's strategy looked like a risky but potentially critical attempt to break the deadlock in the American presidential election campaign, by turning it into a referendum on Mr Bush's turbulent four years in office. The Democratic candidate was also clearly striving to motivate his party's base to get out and vote in large numbers in what is a highly polarised nation.

All week, Democrats have sought to neutralise doubts about Mr Kerry's record of inconsistency on the big issues of national politics over a career in politics spanning two decades. They have insisted that the strength of his character is best attested by his record as a decorated military veteran in Vietnam. Last night, the Democratic candidate was cheered onstage by members of the naval vessel he captained in the Mekong Delta 35 years ago.

But Mr Kerry and his running mate, John Edwards, have also emphasised their determination to prosecute aggressively the war on terrorism. Last night he told the convention: "I defended this country as a young man and I will defend it as president."

He even addressed head-on the accusation that he is too complex a character for the tough decision-making environment of the presidency with another swing at President Bush. "Some criticise me for seeing complexities - and I do - because some issues just aren't all that simple. Saying there are weapons of mass destruction in Iraq doesn't make it so. Saying we can fight a war on the cheap doesn't make it so. And proclaiming mission accomplished certainly doesn't make it so."

The risk in this full-bore strategy unleashed on the President is that, in what was his first nationally televised speech to the nation, he may have presented the image of an overly negative politican, more interested in assailing his opponent than offering an optimistic vision of America's future.

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