BOSTON — Sen. John Kerry challenged President Bush's Iraq policy in blunt, biting terms Thursday 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 a promise that, "Our best days are still to come," and 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 floated gently to the floor.

An hour earlier, the 60-year-old lawmaker made a triumphant entrance into the 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.

Eager to strike out from their convention city, Kerry and vice presidential running mate John Edwards depart Friday for a 3,500-mile, coast-to-coast campaign swing through 21 states.