Mr. Castellanos, who advised Mitt Romney in the 2008 presidential race, confirmed in an interview that he is guiding the group, Rebuilding America Now, which was filed late last week. It is being supported by Tom Barrack, a financier and real estate investor and an old friend of Mr. Trump.

“I’m worried that we don’t have four or eight more years to wait to change direction,” Mr. Castellanos said, explaining his decision to join the group. “This is not about whether the Republican wins or loses the White House. This is about whether we can afford to go in the same direction, and because I’m concerned if we go too much farther we won’t be able to turn back.”…

Mr. Castellanos’s involvement is notable for his previous criticism of Mr. Trump. He had quietly explored the possibilities of creating a “Stop Trump” effort last fall, and he has long urged the Republican Party to change its tone and focus in order to appeal to a broader coalition of voters.

Mr. Castellanos was blunt about what had changed for him. “My choice is now binary. Either I support her, or I support him,” he said. “And the certainty of more of the same in Hillary Clinton is much more dangerous to this country than the uncertainty and disruptive change of Donald Trump.”