Donald Trump has offered his bluntest assessment yet of Nato, calling it “obsolete” and insufficiently geared towards combating international terrorism, the “single biggest threat” in the world.

Taking questions from voters at a CNN town hall broadcast from Wisconsin, which holds its primary elections next Tuesday, the leading Republican candidate doubled down on comments he had made at the weekend to both the New York Times and The Washington Post assailing the Brussels-headquartered military alliance, to which he said the United States was paying too much money.

“Nato has to be changed or we have to do something. It has to be rejiggered or changed for the better,” he said in response to a question from an audience member. He said the alternative to an overhaul would be to start an entirely new organisation, though he offered no details on what that would be.

He also reiterated his concern that the US takes too much of the burden within Nato and on the world stage. “The United States cannot afford to be the policeman of the world, folks. We have to rebuild this country and we have to stop this stuff…we are always the first out,” he offered.

Threatening to take Republican Party even closer to the brink of a nervous breakdown, Mr Trump rowed back on a previous pledge to back whomever becomes its nominee - in the event it is not him. “No, not anymore,” he told the night’s moderator Anderson Cooper, opening up the possibility that he might launch a third-party bid for the presidency if he is not eventually given the Republican crown.

Questioned about his determination to stand by his campaign manager, Corey Lewandowski, who on Tuesday was charged with minor battery in connection with his alleged grabbing of a female reporter, Michelle Fields, at a Trump event in Florida on 8 March, Mr Trump attempted to suggest that she might have presented a real danger at the time.

In a response that attracted scorn on social media, he noted that Ms Fields was carrying a pen, which “could have been a knife, it could have been just a pen which is very dangerous.” He pressed on suggesting even that the Secret Service might have thought her pen was a “little bomb.”