Accusing Democrats of weakness in the war on terror, some in the field also subtly distanced themselves from Republican President George W Bush, as the race hit a new level of intensity five months before first nominating contests.

Rivals Mitt Romney and New York's ex-mayor Rudolph Giuliani declined in the debate in Des Moines, Iowa, to rule out an incursion into remote tribal areas in Pakistan where, according to US intelligence estimates, al-Qaeda fighters are holed up.

"I would take that action if I thought there was no other way to crush al-Qaeda, no other way to crush the Taliban, and no other way to be able to capture bin Laden," said former New York mayor Giuliani.

But Giuliani, who leads nationwide Republican polls, said he hoped to get results by exerting more pressure on Islamabad to crack down on al-Qaeda and the Taliban close to the Afghan border.

"I think Pakistan has, unfortunately, not been making the efforts that they should be making," Giuliani said in the debate televised by ABC.