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Former presidential candidate and Florida Gov. Jeb Bush will not vote for Donald Trump in November, saying the presumptive GOP nominee is "not a consistent conservative" and has not showed the right "strength of character" for the job.

Bush wrote in a Facebook post that the president requires "great fortitude and humility and the temperament and strong character to deal with the unexpected challenges that will inevitably impact our nation in the next four years."

"Donald Trump has not demonstrated that temperament or strength of character," he said. "He has not displayed a respect for the Constitution. And, he is not a consistent conservative. These are all reasons why I cannot support his candidacy."

Bush also wrote that he also won't back Hillary Clinton, whom he described as an "untrustworthy liberal."

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Bush, the son and brother of the nation's last two Republican presidents, was considered the frontrunner for the GOP nomination early in his campaign. He and his allies spent over $80 million on TV ads alone in the presidential campaign but were unable to elevate Bush's candidacy or stop Trump's in its tracks.

Trump fired back at a rally in Omaha, Nebraska Friday afternoon, calling Bush "nasty" and poking fun at his unsuccessful campaign.

"I'm not surprised with the Bush family,” Trump said.

Both former Presidents George W. Bush and George H.W. Bush said they plan to stay on the sidelines during the 2016 campaign.

Another 2016 contender, Lindsey Graham, also said Thursday that he will not support Trump.

Graham, a South Carolina senator who mounted his own unsuccessful presidential campaign last year, said in a series of tweets that he will support neither Trump nor Clinton and will not attend this summer's Republican convention.

I also cannot in good conscience support Donald Trump because I do not believe he is a reliable Republican conservative....(4) — Lindsey Graham (@LindseyGrahamSC) May 6, 2016

.....do not believe he is a reliable GOP conservative nor has he displayed the judgment and temperament to serve as Commander in Chief. (5) — Lindsey Graham (@LindseyGrahamSC) May 6, 2016

I will enthusiastically support Republicans for other offices in South Carolina and throughout the country. (6) — Lindsey Graham (@LindseyGrahamSC) May 6, 2016

Trump issued an even harsher statement ridiculing Graham for his poor performance in the 2016 campaign.

"I fully understand why Lindsey Graham cannot support me. If I got beaten as badly as I beat him, and all the other candidates he endorsed, I would not be able to give my support either," he said in a statement.

"Every time I see Lindsey Graham spew hate during interviews I ask why the media never questions how I single handily destroyed his hapless run for President. As a candidate who did not receive 1% in his own state - compared to my victory at nearly 40% with many others in the race - he has zero credibility."

Graham was one of Trump's harshest critics during the campaign, calling his foreign policy ideas "dangerous" for the country.

He reluctantly supported Ted Cruz's presidential campaign before the Texas senator dropped out.

Graham famously said in January 2016 that choosing between Cruz and Trump would be "like being shot or poisoned."