Campaign advisers to Donald J. Trump, concerned that his focus and objectives had dissolved during the first presidential debate on Monday, plan to more rigorously prepare him for his next face-off with Hillary Clinton. They intend to drill the Republican nominee on crucial answers, facts and counterattacks, and coach him on ways to whack Mrs. Clinton on issues even if he is not asked about them.

Whether he is open to practicing meticulously is a major concern, however, according to some of these advisers and others close to Mr. Trump.

While analysts from both parties and several focus groups declared Mrs. Clinton the winner of the debate, Mr. Trump tried to claim that title for himself, citing unscientific online surveys, and told his advisers that he believed he had done well in the first half-hour of the 90-minute event.

A delicate approach to the candidate is now in the works. Before his advisers can shape Mr. Trump’s performance for the next debate, on Oct. 9 in St. Louis, they need to convince him that he can do better than he did in the first one and that only a disciplined, strategic attack can damage Mrs. Clinton with voters. Advisers said that Mr. Trump had been prepped to handle Mrs. Clinton’s attacks but did not effectively execute responses to them.