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.”
The main focus of the group will be countering the effect of a $90 million advertising offensive planned by Priorities USA Action, the super PAC backing Hillary Clinton. Priorities USA has released two ads, both focused on women, that eviscerate Mr. Trump, including one that uses his own statements against him.
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.