A former nuclear launch officer who served as a national security adviser to Jeb Bush says Donald Trump Donald John TrumpBiden leads Trump by 36 points nationally among Latinos: poll Trump dismisses climate change role in fires, says Newsom needs to manage forest better Jimmy Kimmel hits Trump for rallies while hosting Emmy Awards MORE is "mentally unfit" to be in charge of the nation's nuclear arsenal.

John Noonan, who served for five years as an Air Force nuclear missile launch officer, blasted the Republican presidential nominee for comments indicating his willingness to use nuclear weapons.

"During my years in the Air Force, I worked over 300 nuclear 'alerts'— 24-hour shifts 100 feet below the Wyoming tundra. I sat at my post believing, through both the [George W.] Bush and Obama administrations, that the president was fundamentally rational and would never ask me to do my terrible duty. Not unless the country was in the direst of national emergencies," he wrote in The Los Angeles Times.

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"With Trump as president, the young men and women who are assigned to our nuclear forces will have no such assurances. The very point of nuclear weapons is that they are never used," he wrote.

"I am a Republican, and I have long worked in Republican politics. It gives me no pleasure to say this, but I believe my party’s nominee for president is mentally unfit to assume this heavy responsibility."

Noonan's op-ed follows a series of tweets he sent out last week, after MSNBC host Joe Scarborough said Trump had asked one of his foreign policy advisers three times why nuclear weapons couldn't be used, a report Trump's campaign has denied.

Noonan pointed to other comments Trump has made, such as his suggestion he would nuke the Islamic State in Iraq and Syria and Trump spokeswoman Katrina Pierson asking, "What good does it do to have a nuclear triad if you're afraid to use it?"

"Trump cannot be trusted with weapons that can kill millions. He cannot be handed the nuclear 'football' — a briefcase containing the war plans and codes for our nuclear forces — and be made responsible for its contents," he wrote.

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Noonan, who was also a staff aide at the House Armed Services Committee, said that Trump is "not capable" of making difficult decisions on how to modernize the nation's nuclear weapons. Noonan advised Jeb Bush on security issues before the former Florida governor dropped his own presidential bid in February.

Trump on Saturday hit back at critics, saying he would be the "last one" to ever authorize the use of nuclear weapons.

"Believe me, I would be the last one to use nuclear," Trump said at a rally in Windham, N.H. "Because I understand what it represents. I understand what it means."

Noonan says he doesn't plan to vote for Democratic nominee Hillary Clinton Hillary Diane Rodham ClintonBiden leads Trump by 36 points nationally among Latinos: poll Democratic super PAC to hit Trump in battleground states over coronavirus deaths Battle lines drawn on precedent in Supreme Court fight MORE, either, and is considering voting for Libertarian nominee Gary Johnson Gary Earl JohnsonWhat the numbers say about Trump's chances at reelection Presidential race tightens in Minnesota as Trump plows resources into state The Hill's Campaign Report: Biden condemns violence, blames Trump for fomenting it l Bitter Mass. primaries reach the end l Super PAC spending set to explode MORE or writing in Kirk Cousins, the Washington Redskins quarterback.