Ex-McCain aide: Trump has 'unstable personality'
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A former top aide to Sen. John McCainJohn Sidney McCainMichelle Obama weighs in on Trump, 'Squad' feud: 'Not my America or your America. It's our America' Meghan McCain shares story of miscarriage Media cried wolf: Calling every Republican a racist lost its bite MORE (R-Ariz.) said that presumptive GOP presidential nominee Donald TrumpDonald John TrumpLiz Cheney: 'Send her back' chant 'inappropriate' but not about race, gender Booker: Trump is 'worse than a racist' Top Democrat insists country hasn't moved on from Mueller MORE has an "unstable personality."

“He’s going to lose, and I think he’s got kind of an unstable personality to begin with,” Mark Salter said during Politico's "Off Message" podcast this week.

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Salter, a speechwriter for McCain's 2008 presidential campaign, last week tweeted he would back Democratic front-runner Hillary ClintonHillary Diane Rodham ClintonGeorge Takei: US has hit a new low under Trump Democrats slam Puerto Rico governor over 'shameful' comments, back protesters Matt Gaetz ahead of Mueller hearing: 'We are going to reelect the president' MORE over Trump. His comments came after Trump cited a National Enquirer story that claimed Sen. Ted CruzRafael (Ted) Edward Cruz2 Republican senators introduce resolution to label antifa as domestic terrorists Ted Cruz: Trump's chances of winning reelection are '50-50' How to reduce Europe's dependence on Russian energy MORE's (R-Texas) father was linked to Lee Harvey Oswald before the assassination of President John F. Kennedy.

"The GOP is going to nominate for President a guy who reads the National Enquirer and thinks it's on the level. I'm with her," he tweeted last week.

During Politico's podcast, Salter predicted Trump would "come apart ... in some kind of visible way.

“I think that's quite possible. ... I'm not a psychiatrist, but there is something wrong with [the] guy,” he said.

Even though Salter said he plans to support Clinton over Trump, he said he likely would be "disappointed" — but he emphasized that her presidency would still be an improvement. 

“I’m sure 10 minutes into Hillary Clinton’s inaugural address I’ll sort of be disappointed,” he said.

“[But] her foreign policy would probably be a modest to maybe substantial improvement over the incumbent. ... I’ve never voted for a Democrat for president in my life. But she is the more conservative choice and the least reckless one. She won’t — or at least she hasn’t said that she would — take the U.S. out of NATO. She hasn’t encouraged other nations to get their own nuclear weapons. She hasn’t threatened to slap a 45 percent tariff on imported goods.”

Salter said there's essentially nothing the presumptive GOP nominee can do to change his mind.

"He'd have to have a brain transplant," he said, when asked.