Mr. Trump also attacked Mr. Romney, calling him “a choker” and saying he “didn’t work like he should have worked” when he was the nominee in 2012. Once Mr. Romney lost, Mr. Trump added, he should have gone “off into the sunset.”

“You don’t sit there jealous and sick to your stomach,” he said.

A day earlier, Mr. Romney, in an interview with CNN’s Wolf Blitzer, said he feared that Mr. Trump’s election would lead to “trickle-down racism” that would “change the character of the generations of Americans that are following.”

Reince Priebus, the chairman of the Republican National Committee, gently chided Mr. Romney on Twitter on Friday, writing, “Respect Mitt and differences but couldn’t disagree more.”

Adding that the Supreme Court was “too important to lose for generations,” Mr. Priebus ended his post, “Let’s stop this and unify.”

Mr. Trump and the party have gotten a late start on fund-raising for the general election and need as many of the party’s reliable donors as they can attract. At Mr. Romney’s donor retreat on Saturday, Mr. Priebus was more pointed, telling donors opposed to Mr. Trump that the party would “win with or without you,” according to an attendee present for his remarks.

A representative for Ms. Whitman did not respond Saturday to a request for an interview about her comments. Ms. Whitman, according to one of the people present, did not stop at comparing Mr. Trump to Hitler and Mussolini. She also warned the gathering that if Republicans compromised on their principles to win an important election, they would be entering fraught territory.

“What happens next time?” she asked, implying that it could lead to more compromises and more candidates like Mr. Trump.