ALEXANDRIA, Virginia, April 1, 2008 (AFP) - Republican presidential candidate John McCain is waging a biting counter-attack against Democratic attempts to brand him as a champion of multi-generational 100-year war in Iraq.

The Arizona senator is trying to defuse a potentially potent line of attack for whichever Democrat he faces in November's presidential election, as the unpopular war stretches into its sixth year, with 4,000 US soldiers dead.

He is pivoting from attacks on his Iraq stance, to portray Democratic pace-setter Barack Obama, especially, as a dangerous neophyte who cannot be entrusted with command of the US armed forces.

'In all due respect, he does not understand the fundamental elements of national security and warfare,' McCain told reporters on his campaign plane, Monday, pushing back against the latest Obama broadside on the war.

McCain, campaigning in Virginia Tuesday, is stepping up efforts to maneuver out of a tight spot, partly of his own making, arising from his support for the war, and President George W. Bush's troop surge policy.

In January, the Arizona senator said during an event in New Hampshire that it would be 'fine' with him if US troops were in Iraq for 100 years 'as long as Americans are not being injured or harmed or wounded or killed.'

McCain argued that the US presence in Iraq could eventually mirror peacekeeping and stability missions carried out by tens of thousands of US troops in Japan and South Korea for more than half a century.

McCain has a problem of political perception however. Such statements, even taken out of context, are exactly the kind of timebombs that can damage a general election campaign, and fuel hours of negative advertising.

His potential Democratic foes are increasingly exploiting the comment to argue that McCain is bombastic and wants a 100 year war in Iraq, and shares Bush's desire for a long-term boots-on-the-ground presence.

Obama, who has vowed to end the Iraq war in 2009, if he is elected, stirred the row again on Monday during a campaign swing through Pennsylvania.

'I don't think it's unfair at all. John McCain -- I mean, we can run the YouTube spot -- has said that we will stay there as long as it takes and, if it takes another 100 years, he's up for that commitment,'

'The problem that we've had, both with John McCain and George Bush, is there's no clear definition of success. There never was. And that's why this has been such a profound strategic error.

McCain equated withdrawal with 'surrender,' Obama said, which implied 'we will be there as long as he thinks it's necessary for us to be there.'

The Arizona senator's biting comeback raised the prospect of a fierce debate on Iraq, should Obama finally emerge as Democratic nominee, though he said he believed Americans are smart enough to see through Democratic attacks.

'In all due respect, (Obama) displays a fundamental misunderstanding of history and how we've maintained national security, and what we need to do in the future to maintain our security in the face of the transcendent challenge of radical Islamic extremism.

'I understand that because he has no experience or background in any of it.'

Obama spoke out against going to war in Iraq before he was elected to the Senate, and has tried to use that stance to show his judgement is superior to that of rival Hillary Clinton, who voted to authorize the invasion.

In a major foreign policy address on March 17, Clinton also tried to hang the 100-year albatross around McCain's neck.

'Let's be clear: withdrawal is not defeat,' she said.

'Defeat is keeping troops in Iraq for 100 years, defeat is straining our alliances and losing our standing in the world, defeat is draining our resources and diverting attention from our key interests.'

McCain has frequently said he would rather lose a political campaign than a war, in an implicit admission that a sharp deterioration in post-surge conditions in Iraq could scupper his White House hopes.

Last week, the presumptive Republican nominee signalled he was not backing down, setting a bar for success in Iraq many war critics believe cannot be reached.

'Success in Iraq and Afghanistan is the establishment of peaceful, stable prosperous, democratic states that pose no threat to neighbors and contribute to the defeat of terrorists.

'It is the triumph of religious tolerance over violent radicalism.'

McCain also warned that America had incurred a 'moral responsibility in Iraq.'

'It would be an unconscionable act of betrayal, a stain on our character as a great nation, if we were to walk away from the Iraqi people,' he said.