It was not, Mr. Rubio conceded to them, his best performance. But only after the senator scrolled through Twitter — flooded with brutal, mocking reviews — did he fully grasp the damage he had done to his own campaign.

His aides and supporters, on the other hand, had been hearing it all night. “Shocked” was how one supporter close to the campaign leadership described the reaction. The emails pouring in from donors were incredulous. Why did he not fight harder against Gov. Chris Christie of New Jersey, who had mercilessly mocked him? Why did he keep repeating the same talking points? Why was he sweating so much?

At his campaign headquarters in Washington, some of the younger staff members were so deflated that senior advisers met with them on Sunday morning to reassure them the episode was just a hiccup — the kind that happens all the time in presidential races.

But privately, advisers acknowledged, this was different — and avoidable. Mr. Christie had not just telegraphed the coming attack, he directly forewarned Mr. Rubio backstage on Saturday night as the two men waited for their names to be called by the ABC News moderators. “I understand I am going to have a hard time tonight,” Mr. Rubio playfully told Mr. Christie.