"Donald Trump has not demonstrated that temperament or strength of character. 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 wrote in a Facebook post Friday afternoon.
Bush noted that he won't vote for Democratic front-runner Hillary Clinton either.
"In November, I will not vote for Donald Trump or Hillary Clinton, but I will support principled conservatives at the state and federal levels, just as I have done my entire life," he said. "For Republicans, there is no greater priority than ensuring we keep control of both chambers of Congress. I look forward to working hard for great conservatives in the Senate and House in the coming months."
Trump was relentless in going after Bush during the presidential campaign. Early on, he branded the former governor as "low energy," a devastating characterization he was never able to shake. Trump also went after Bush's Mexican-born wife at one point, saying Bush liked "Mexican illegals because of his wife."
Sen. Lindsey Graham (R-S.C.), who endorsed Bush after he dropped his own presidential bid, also said this week that he won't be backing Trump. The real estate mogul issued a statement Friday essentially saying he didn't care.
"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," Trump said, adding, "While I will unify the party, Lindsey Graham has shown himself to be beyond rehabilitation. And like the voters who rejected him, so will I!"
Right-leaning groups are also starting to throw in the towel on the presidential race. In a Wednesday blog post titled "What Now?", CatholicVote.org said it won't support Trump and instead will focus on "4-5 critical Senate races along with as many as 10+ House seats." Similarly, Club for Growth President David McIntosh lamented that Trump is not "a genuine pro-growth conservative" and said the group will focus its attention on House and Senate races this year.
On Thursday, House Speaker Paul Ryan (R-Wis.) said he was "just not ready" to support Trump -- although he left open the possibility that he would be comfortable doing so at some point. Trump is scheduled to meet with House GOP leaders in Washington on Thursday, when he and Ryan will also meet separately.
Read Bush's full Facebook post: