Donald J. Trump was in a state of shock: He had just fired his campaign manager and was watching the man discuss his dismissal at length on CNN. The rattled candidate’s advisers and family seized the moment for an intervention.

Joined by his daughter Ivanka and her husband, Jared Kushner, a cluster of Mr. Trump’s confidants pleaded with him to make that day — June 20 — a turning point.

He would have to stick to a teleprompter and end his freestyle digressions and insults, like his repeated attacks on a Hispanic federal judge. Paul Manafort, Mr. Trump’s campaign chairman, and Gov. Chris Christie of New Jersey argued that Mr. Trump had an effective message, if only he would deliver it. For now, the campaign’s polling showed, too many voters described him in two words: “unqualified” and “racist.”

Mr. Trump bowed to his team’s entreaties, according to four people with detailed knowledge of the meeting, who described it on the condition of anonymity. It was time, he agreed, to get on track.

Nearly two months later, the effort to save Mr. Trump from himself has plainly failed. He has repeatedly signaled to his advisers and allies his willingness to change and adapt, but has grown only more volatile and prone to provocation since then, clashing with a Gold Star family, making comments that have been seen as inciting violence and linking his political opponents to terrorism.