Republican John McCain and Democrat Barack Obama have clashed over whether Iraq would be prey for al-Qaeda if US troops are withdrawn.
Mr McCain and Mr Obama looked past Hillary Clinton during a debate in Texas and quarrelled anew over the war in Iraq as it approaches its fifth anniversary in March.
Senator John McCain
Mr McCain, who has linked his candidacy to a successful outcome in Iraq, attacked Mr Obama's stance on the war at the town hall meeting.
Mr Obama said during the debate with Mrs Clinton that once he withdrew US troops from Iraq, if al-Qaeda were to form a base there, "then we will have to act in a way that secures the American homeland and our interests abroad."
"I have some news," McCain said. "Al-Qaeda is in Iraq. It's called al-Qaeda in Iraq. My friends, if we left, they wouldn't be establishing a base, they'd be taking a country ,and I'm not going to allow that to happen."
Mr Obama hit back at a rally in Columbus, Ohio, saying Mr McCain had joined President George W Bush in supporting a war "that should have never been authorised and should have never been waged."
"I have some news for John McCain , and that is that there was no such thing as al-Qaeda in Iraq until George Bush and John McCain decided to invade Iraq," he said.
He mocked Mr McCain for his oft-repeated remark that he will get al-Qaeda leader Osama bin Laden if he has to follow him to the "gates of hell."
"So far all he's done is follow George Bush into a misguided war in Iraq," Mr Obama said.
Mrs Clinton was pushing economic themes in Ohio, a state that has lost 23 per cent of its manufacturing jobs since 2000 and where the subprime mortgage crisis has hit hard, with foreclosures climbing 88 per cent in 2007.
Elsewhere, New York Mayor Michael Bloomberg appeared to end long-running speculation he was considering an independent bid for the presidency. Writing in Thursday's New York Times, he said he may endorse someone else.