Jeb Bush mega-donor backs Hillary Clinton
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One of the biggest Republican donors in this presidential cycle has swung his support forcefully behind Democrat Hillary ClintonHillary Diane Rodham ClintonHillary Clinton slams Trump rally: 'The time has come again' to fight for democracy Trump blasts minority Democrats, rally crowd chants 'send her back' The Memo: Democrats debate Trump response – 'Being righteous and losing sucks' MORE, saying a Donald TrumpDonald John TrumpAmash responds to 'Send her back' chants at Trump rally: 'This is how history's worst episodes begin' McConnell: Trump 'on to something' with attacks on Dem congresswomen Trump blasts 'corrupt' Puerto Rico's leaders amid political crisis MORE presidency would destroy America.

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Florida billionaire Mike Fernandez spent $3 million boosting Jeb Bush in the GOP primaries and even gave $100,000 last year to America Rising, a Republican group committed to destroying Clinton.

But in the opinion section of Thursday's Miami Herald, Fernandez outlines what he frames as a patriotic and moral duty for all Republicans to abandon Trump and vote Clinton.

He even questions Trump's mental health.

"As a Republican who has contributed millions of dollars to the party’s causes, I ask: Why has our party not sought a psychological evaluation of its nominee?”

Fernandez adds that a Trump presidency could destroy the republic as it was conceived by the founders.

"There is no basis in thinking that our democracy is so strong, our checks and balances so finely hedged, that no single person can lead us off the precipice," Fernandez writes.

"Trump can."

Fernandez also scoffs at the "excuse" of party loyalty being pushed by some members of the Republican establishment who believe that Trump, though far from perfect, would be clearly preferable to Clinton.

"No longer can we hide behind the excuse that party loyalty is paramount, and that a bad candidate of our own is always better than any candidate of theirs," Fernandez writes.

"Blind loyalty in this case is the ultimate definition of disloyalty to our beliefs. Loyalty to our nation must be the ultimate arbiter of our choice."

Fernandez was early to declare Trump as unfit and a threat to democratic values.

Late last year, while still supporting Bush in the Republican primaries, Fernandez bought newspaper ads attacking Trump in Miami, Las Vegas and Des Moines, Iowa.

In his Thursday opinion article, Fernandez writes he has now "arrived at this difficult moment."

"I harbor no illusion that Hillary Clinton is perfect; none of us is. I do not see eye to eye on some issues with the former senator from New York. However, Clinton is, without doubt, a superior choice to Donald Trump."

Fernandez concludes his piece by addressing his "fellow Republicans":

"Swallow hard," he tells them, "look into your heart — and your gut.

"Vote for Hillary Clinton and then every single Republican on the ticket."