Trump: ‘I mean had Andrew Jackson been a little later you wouldn’t have had the Civil War. He was a very tough person, but he had a big heart. He was really angry that he saw what was happening with regard to the Civil War, he said, “There’s no reason for this.” ’

Jackson died in 1845. The Civil War broke out in 1861.

Had Jackson been alive at the start of the Civil War, Mr. Meacham said, it would be difficult to predict his reaction. It would have brought his commitment to the Union into conflict with his identity as an unapologetic slave owner. Mr. Jackson was from Tennessee, which fought for the Confederacy. Mr. Trump visited his tomb there this year.

But any president would have had to contend with the South’s attempt to expand the institution of slavery into territory newly acquired by the United States. It’s what Mr. Meacham called the unavoidable historical question.

“The expansion of slavery caused the Civil War,” he said. “And you can’t get around that. So what does Trump mean? Would he have let slavery exist but not expand? That’s the counterfactual question you have to ask.”