John Noonan, a national-security expert who advised Jeb Bush and Mitt Romney, went off on Twitter on Wednesday, arguing that Trump's position on nuclear weapons makes him unfit for the presidency.

A foreign-policy expert heard Trump question the US's long-standing policy to avoid using nuclear weapons, MSNBC host Joe Scarborough reported on Wednesday.

Trump reportedly asked about nuclear weapons three times, saying at one point, "If we have them, why can't we use them?"

Trump's campaign manager, Paul Manafort, later declared that Scarborough's story was "absolutely not true."
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