Frank Lavin, Reagan's political director from 1987 to 1989, said he has voted Republican in presidential elections for 40 years, but will cast his ballot for Democratic nominee Hillary Clinton this year because "she's just got a better approach to America's role in the world."
In recent weeks, dozens of Republicans including former national security officials have said they will not support Trump.
Lavin, now CEO at trade consulting firm Export Now, also served as an undersecretary in the Commerce Department and ambassador to Singapore under President George W. Bush. He said it is difficult for Republican politicians to break with Trump, but for him, the choice to support Clinton over his party's candidate was easy, citing the twin issue of his rhetoric and record of bankruptcies.
"If he's not actually a bigot, he certainly consorts with bigotry and plays to that sentiment," Lavin said.
Trump drew accusations of bigotry early in his campaign. When he announced his bid, he said many Mexican immigrants were criminals and rapists. He later angered fellow Republicans by saying a Mexican-American judge presiding over a lawsuit against Trump University could not be impartial because of his heritage.
The GOP nominee has said he would ban Muslims from entering the United States following a series of terrorist attacks in the United States and Europe orchestrated or inspired by Islamic State militants. Trump has slipped in the polls after publicly feuding with the father of a fallen Muslim-American Army captain who spoke out against him at the Democratic National Convention.
Lavin said "that denigration of Muslims, of Mexicans and so forth, I thought that's unbecoming, and there should be no place for that kind of bigotry in public life. There should be no place for that kind of bigotry in our hearts."
Trump's trips to bankruptcy court show there "is something terribly wrong with this fellow's business judgment, his management, maybe even his ethics, if he's got four bankruptcies like that and he's still scooting around in a private jet," Lavin added.
Four of Trump's hotel and casino companies filed for bankruptcy between 1991 and 2009.
Trump has said bankruptcy protection is a common business practice, and he claims he exited Atlantic City, New Jersey, as the gambling destination entered decline. The New York Times and other news outlets have reported he funded his Atlantic City properties with unsustainable debt and his casinos underperformed competitors long before the city's fortunes turned.
Trump campaign officials did not immediately respond to CNBC's request for comment.
Lavin said he previously worked with Clinton on New York's Olympic bid and a past Shanghai Expo and called her capable, bright, diligent, and hard-working. He said he was disappointed she withdrew her support for a trade deal among 12 Pacific Rim nations.
Critics have alleged Clinton's past support for the Trans-Pacific Partnership as secretary of state shows she is flip-flopping to score political points with liberal voters.
"I'll tell you no matter where she is on trade, and moving American companies ahead, Donald Trump is that much worse," Lavin said.