President Bush and John F. Kerry clashed over the Iraq war in an intense and substantive first debate, in which the Democratic nominee charged that the war was a diversion from the more important war against al Qaeda and the president defended the conflict as crucial to the nation's security.

In their first such encounter, the two presidential candidates repeatedly returned to the themes that have dominated the campaign. The Massachusetts senator accused the president of "misleading" the nation as he went to war, while Bush said nine times that Kerry's "mixed messages" and "mixed signals" mean he does not have the steadiness to be an effective commander in chief.

"The president has made, I regret to say, a colossal error of judgment, and judgment is what we look for in the president of the United States of America," Kerry said of the war in Iraq. "I would not take my eye off of the goal: Osama bin Laden."

Bush countered that Kerry's criticism of the war in Iraq would make it impossible for him to lead allies to victory there. "What's the message going to be: Please join us in Iraq for a 'grand diversion'?" Bush asked. Allies, he said, "are not going to follow somebody who says this is the wrong war at the wrong place at the wrong time. They're not going to follow somebody whose core convictions keep changing because of politics in America."

There were no glaring mistakes by either candidate during the 90-minute debate at the University of Miami, although Bush often appeared agitated, scowling at times as Kerry leveled his charges. While both delivered their messages forcefully, Kerry sharply questioned the president's credibility and highlighted his own ability to serve as commander in chief.

-- Dana Milbank and Jim VandeHei

In Miami, President Bush said Iraq is a key part of the war on terrorism, but John F. Kerry charged Bush made "a colossal error of judgment" by shifting attention from al Qaeda.