Mr. Trump, for his part, sought to blame everything but himself. During an appearance on Fox News on Tuesday, he charged that the moderator, Lester Holt of NBC News, had become overly aggressive with him — although he inaccurately said that Mr. Holt had questioned him over a 1973 federal discrimination lawsuit against Mr. Trump’s company. (Mrs. Clinton had raised the lawsuit question.) He also suggested that his performance was related to a faulty mike — even though he was perfectly audible during the telecast — and that he may have been the victim of sabotage.  And at a rally in Florida on Tuesday night, he ripped Mrs. Clinton in scathing terms that he declined to use when they were face to face.  But Mr. Trump’s lack of facility as a one-on-one debater was glaring at times on Monday, such as his inability to challenge Mrs. Clinton’s judgment over the attacks on the American diplomatic mission in Benghazi, Libya. He protested on Fox News that he could not find a way to bring up Benghazi, saying, “Don’t forget, you are asked a question as to progress or as to something, and it’s hard to get off to Benghazi sometimes the way the questions were framed.”  Mrs. Clinton, who prepared at length for the debate, was far more deft at unnerving her opponent, finding a way during an exchange about trade to bring up a loan that Mr. Trump had received from his father. Topics during general election debates are often inserted at prime openings by the candidates themselves, rather than by the moderators, whom Mr. Trump relied on during the primary debates to set the tone.  The shape and schedule for Mr. Trump’s next round of debate preparations are still under discussion, his advisers said.  Some of the advisers want to practice getting under his skin, as Mrs. Clinton did, to gauge his response, but they offered no details about doing so. Others wanted practice sessions built around the next debate’s format, a town-hall-style meeting, where Mr. Trump is likely to engage with undecided voters asking him questions and, at times, move from his chair to walk the stage. Mr. Trump has little experience with the format, which can be challenging for people who do not practice managing their body language and movements.  Several advisers also want to impress upon him the need to stick to a strategy and a plan of battle against a female candidate — the kind of opponent he has less experience facing — rather than spend time polishing a string of disparate zingers that Mrs. Clinton, a skilled debater, was able to easily parry Monday night.  Mrs. Clinton succeeded several times in baiting Mr. Trump, making him become defensive, lose his cool or dig himself into a political hole, particularly late in the debate as he struggled to defend himself against charges that he had made sexist and racist remarks. He also repeatedly interrupted or talked over Mrs. Clinton, which some female voters found alienating. Some allies of Mr. Trump say he is not preparing enough to do battle with a woman in mind; he has only one senior adviser who is a woman, his campaign manager, Kellyanne Conway.  Almost all of his advisers rejected the idea that the debate was a failure for Mr. Trump, noting that he landed some punches and insisting that Mrs. Clinton looked more polished than she was because of her opposition.  But all of them described the debate as filled with missed opportunities. And they openly expressed frustration that Mr. Trump seemed unable to stop chasing chum that Mrs. Clinton tossed at him.  Mr. Trump’s debate preparation was unconventional. Aides have introduced a podium and encouraged him to participate in mock debates, but he has not embraced them, focusing mostly on conversations and discussions with advisers.  During the primaries, the group briefing him for debates was small and closely held. By the weekend before the debate on Monday at Hofstra University, there were nearly a dozen people preparing Mr. Trump, including the retired Army generals Michael Flynn and Keith Kellogg, neither of whom has experience in presidential debates.  There were early efforts to run a more standard form of general election debate-prep camp, led by Roger Ailes, the ousted Fox News chief, at Mr. Trump’s golf course in Bedminster, N.J. But Mr. Trump found it hard to focus during those meetings, according to multiple people briefed on the process who requested anonymity to discuss internal deliberations. That left Mr. Ailes, who at the time was deeply distracted by his removal from Fox and the news media reports surrounding it, discussing his own problems as well as recounting political war stories, according to two people present for the sessions.  Rudolph W. Giuliani, the former New York City mayor and a friend of Mr. Trump’s who has been traveling with him extensively, took over much of the preparation efforts by the end. But with Mr. Trump receiving so much conflicting advice in those sessions, he absorbed little of it.  The team had primed Mr. Trump to look for roughly a dozen key phrases and expressions Mrs. Clinton uses when she is uncertain or uncomfortable, but he did not seem to pay attention during the practice sessions, one aide said, and failed to home in on her vulnerabilities during the debate.  “It clearly looked like he ran out of gas after 30 minutes, and that came through loud and clear,” said Scott Reed, the senior political strategist for the U.S. Chamber of Commerce, who has not supported Mr. Trump.  “Trump needs to show a higher level seriousness, so that he’s better positioned as an agent of change,” Mr. Reed added. “If he can accomplish that, he’ll win undecided voters and late-breaking voters who clearly don’t want to support Hillary Clinton.”  Mr. Trump’s best lines, several aides said, came when he talked about how Mrs. Clinton had been in politics for 30 years and had yet to offer any solutions, and when he parried a question from her about releasing his tax returns by saying he would release them when she released the 33,000 deleted emails from her time as secretary of state. But he quickly dropped the emails line.  The aides were particularly frustrated when Mr. Trump, unprompted, mentioned the comedian Rosie O’Donnell during an exchange about his treatment of women, saying she deserved his vitriol and criticism.  Mr. Trump’s campaign tried to soothe supporters on a conference call on Tuesday by pointing to instant polls that showed him winning the debate, even though a vast majority of surveys showed that Mrs. Clinton led. And his backers painted an optimistic picture of a campaign on course.  “I think he was extremely effective in speaking to the targeted audience of swing state voters and undecided voters,” said John Jay LaValle, the chairman of the Republican Party in Suffolk County, N.Y., and a supporter of Mr. Trump.
Advisers plan to more rigorously prepare him for his next debate with Hillary Clinton, but whether he is open to meticulous preparation is a major concern.