"I know that for many of you sitting at home, parents of kids in Iraq, you want to know who's the person who could be a commander in chief, who can get your kids home and get the job done and win the peace," Kerry said Thursday, during the first nationally televised debate between the candidates.

The event at the University of Miami saw the two rivals slug it out as they vied to convince viewers over who has the better character to lead the nation and head off a repeat of the September 11, 2001 terrorist attacks.

"You cannot lead if you send mixed messages. Mixed messages send the wrong signals to our troops. Mixed messages send the wrong signal to our allies," the incumbent Republican president charged.

"He changes positions on the war in Iraq," Bush said of Kerry.

The Republicans have spent millions of dollars on campaign ads in recent months portraying Kerry as indecisive. Bush has criticised what he says are Kerry's multiple "nuances" on Iraq.

Central to Bush's position is the argument the veteran Massachusetts senator cast a vote in Congress in 2002 authorising the president to go to war against Iraq and Saddam Hussein, but has since lambasted the war.

Kerry, for his part, has slammed Bush for being too stubborn and out of touch on key issues.

"What I worry about with the president is that he's not acknowledging what's on the ground. He's not acknowledging the realities of North Korea, he's not acknowledging the truth of the science of stem cell research or of global warming and other issues. And certainty sometimes (this) can get you in trouble," Kerry countered.

Democrats say Bush's stubborn desire to topple former Iraqi dictator Saddam Hussein has diverted vital US resources from what they say is the real war on terror: Al-Qaeda and the hunt for its leader, Osama bin Laden.

They say Bush's decision to invade Iraq has reaped chaos, cost American lives and money, and that Bush's pre-invasion justifications for the war, weapons of mass destruction, have never been proved.

Each man claims to have a better character than his opponent.

"I've never wavered in my life. I know exactly what we need to do in Iraq and my position has been consistent," Kerry, a decorated Vietnam war veteran, said.

"I defended this country as a young man in war and I will defend it as president of the United States," he stressed.

Bush said he was an instinctive leader who sticks to his guns.

"Of course, we change tactics when need to but we never change our beliefs," Bush said.

Both camps claimed their candidate would be the better commander in chief after the first debate of three ended.

White House communications director Dan Bartlett said: "I think he (Bush) spoke from the heart and spoke with strength about the necessity for our country to fight the terrorists over there so we don't have to face them here at home."

Kerry's running mate, Senator John Edwards, countered that "People saw the next commander in chief."

"This man (Kerry) is so ready to keep this country safe and to lead America, and the American people saw that last night."

American voters will have the ultimate say when they cast their ballots on November 2.

All rights reserved. copyright 2018 Agence France-Presse. Sections of the information displayed on this page (dispatches, photographs, logos) are protected by intellectual property rights owned by Agence France-Presse. As a consequence, you may not copy, reproduce, modify, transmit, publish, display or in any way commercially exploit any of the content of this section without the prior written consent of Agence France-Presse.