A former top aide to Sen. John McCain John Sidney McCainMcSally says current Senate should vote on Trump nominee Say what you will about the presidential candidates, as long as it isn't 'They're too old' The electoral reality that the media ignores MORE (R-Ariz.) said that presumptive GOP presidential nominee Donald Trump Donald John TrumpObama calls on Senate not to fill Ginsburg's vacancy until after election Planned Parenthood: 'The fate of our rights' depends on Ginsburg replacement Progressive group to spend M in ad campaign on Supreme Court vacancy 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 Clinton Hillary Diane Rodham ClintonWhat Senate Republicans have said about election-year Supreme Court vacancies Bipartisan praise pours in after Ginsburg's death Trump carries on with rally, unaware of Ginsburg's death MORE over Trump. His comments came after Trump cited a National Enquirer story that claimed Sen. Ted Cruz Rafael (Ted) Edward CruzSenate Republicans face tough decision on replacing Ginsburg Cruz: Trump should nominate a Supreme Court justice next week Renewed focus on Trump's Supreme Court list after Ginsburg's death 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.