“At some point, you have to look in the mirror and recognize that you cannot possibly justify support for Trump to your children — especially your daughters,” said David Humphreys, a Missouri business executive who contributed more than $2.5 million to Republicans from the 2012 campaign cycle through this spring.

Bruce Kovner, a New York investor and philanthropist who with his wife has given $2.7 million to Republicans over the same period, was just as blunt. “He is a dangerous demagogue completely unsuited to the responsibilities of a United States president,” Mr. Kovner wrote in an email, referring to Mr. Trump.

“Even for loyalists, there is a line beyond which the obvious moral failings of a candidate are impossible to disregard,” he wrote. “That line has been clearly breached.”…

“The R.N.C. long ago should have cut ties with Donald Trump,” said William E. Oberndorf, a California investor who has given more than $3 million to Republicans since 2012. “Reince should be fired and replaced with someone who has the competence and leadership skills to rebuild the R.N.C.”