One was his debate performance. On the flight home from the debate, he described it as superb to family and staff, who said little to dissuade him. But by the next day, Jared Kushner, Mr. Trump’s senior adviser and son-in-law, was imploring friendly media personalities and other allies to level with his father-in-law about the angry and aggressive debate style he had displayed, according to people familiar with the calls. Polls would soon show the performance had cost the president support…

Some advisers have urged him to rethink his debate-preparation strategy, but, as of Wednesday afternoon, Mr. Trump hadn’t attended any prep sessions for Thursday’s debate, formally or informally, people familiar with the matter said…

Mr. Trump is publicly optimistic, but privately appears aware that he is trailing in the race, people familiar with the matter said. He has attempted to establish order in several unusual ways. He pulled out of a scheduled debate in Miami last week after organizers changed the format to make it a virtual event. With the election weeks away, he has asked some advisers about shaking up his re-election team, seeking opinions on campaign manager Bill Stepien and Jason Miller, a senior campaign adviser who holds significant influence on the paid advertising and messaging…

Tuesday brought the first debate, which historically often has been tricky for incumbents, who are typically more out of practice than their rivals who have spent months honing debate skills during the primary. Mr. Trump eschewed traditional practice debate sessions in favor of more informal discussions with his team, advisers said. At one point during practice, he asked the group, “Why are you guys in here? Why aren’t you out there defending me on TV?”