BOSTON (AP) -- Sen. John Kerry challenged President Bush's Iraq policy in blunt, biting terms last night and promised cheering Democratic National Convention delegates, 'I will be a commander in chief who will never mislead us into war.'

'America can do better, and help is on the way,' the Democratic presidential nominee vowed over and over in a prime-time acceptance speech that marked the beginning of the general election phase of his long quest for the White House.

'Saying there are weapons of mass destruction in Iraq doesn't make it so,' said the four-term Massachusetts senator, a decorated Vietnam War veteran battling an incumbent president in an age of terrorism.

'Saying we can fight a war on the cheap doesn't make it so. And proclaiming mission accomplished certainly doesn't make it so,' Kerry told thousands of delegates packed into an overheated, overcrowded FleetCenter as well as a nationwide television audience of uncounted millions.

The hall erupted in cheers as Kerry completed his speech with an optimistic, Reaganesque prediction that 'our best days are still to come.' The ovation grew louder when running mate John Edwards joined Kerry on the podium, followed by their wives and children.

Moments later, tens of thousands red, white and blue balloons and strips of confetti beyond counting floated gently to the floor.

It was a boisterous, jubilant send-off for the Democratic ticketmates, who depart their convention city today for a 3,500-mile, coast-to-coast campaign swing through 21 states.

After spending the week at his Texas ranch, Bush also resumes campaigning this weekend, with a bus tour of battleground states and a new message. 'We have turned the corner, and we are not turning back,' says his new stump speech, excerpts of which were obtained by The Associated Press.

Bush's challenger made a triumphant entrance into the Democrats' convention hall for the most important speech of his political life, walking a happy gantlet of delegates who reached out eagerly to greet him. 'I'm John Kerry and I'm reporting for duty,' he said moments later from the podium -- and snapped off a salute.

In a speech salted with frequent references to patriotism and service, Kerry said the American flag doesn't 'belong to any president. It doesn't belong to any ideology and it doesn't belong to any political party. It belongs to all the American people.'

The Democratic challenger's speech capped a four-day convention designed to persuade millions of undecided voters in the battleground states that he is a man tested by war and ready to assume command.

On the last evening, as on all others, nothing was left to chance -- from a new campaign video designed to show Kerry's softer side to a brief tribute from fellow Vietnam veteran Jim Rassmann.

'John Kerry saved my life,' he said simply.

The video provided the first reference from the convention podium to Kerry's emergence as a prominent anti-war activist more than three decades ago after he returned home from Vietnam.

But Republicans were quick to point out that the evening program omitted one detail in Kerry's resume -- his election as lieutenant governor under Massachusetts Gov. Michael Dukakis, a liberal who blew a huge lead in the 1988 presidential race and wound up losing to Bush's father.

Kerry began the week tied or slightly ahead of Bush in the polls, a strong position for a challenger. Whatever sort of surge in support he receives from four days of his highly choreographed convention, Republicans hope to counter next month when they meet in New York to nominate Bush and Vice President Dick Cheney for re-election.

In his speech, Kerry painted a portrait of a nation suffering economically after four years of Republican rule.

'Wages are falling, health care costs are rising and our great middle class is shrinking. People are working weekends; they're working two jobs, three jobs and they're still not getting ahead,' he said.

'We can do better and we will. We're the optimists,' he said, and added, 'We value an America where the middle class is not being squeezed, but doing better.'

Kerry's decision to question the president over Iraq comes at a time when Bush is struggling to reverse a decline in support for his policies in a conflict that has claimed more than 900 lives, many of them since the president stood on an aircraft carrier beneath a banner that proclaimed 'Mission Accomplished.'

A Pew Research Center survey earlier this month showed 42 percent support for Bush on the war, down from 59 percent six months earlier.

But Kerry expanded his criticism far beyond Iraq as he sought to draw a contrast with the president on the national security issues he has placed at the core of his challenge for the White House. Kerry vowed to reverse policies that send U.S.