What does it mean to stand behind a bigoted, undisciplined President—who is prone to irrational outbursts and liable, on any given day, to lash out at those around him—as he campaigns for his favored candidates in the midterm elections? Any member of the Trump Administration or the military who is wondering about that question, at a time when the President is gearing up with rallies at which he calls officials to his side and railing against immigrants and his enemies, might pick up Ron Chernow’s recent book, “Grant,” and turn to Chapter 26, “A Swing Around the Circle.” That title comes from the nickname of a tour of the Midwest that President Andrew Johnson, who had taken office after the assassination of Abraham Lincoln, embarked on in late August, 1866. Ulysses S. Grant was then the Commanding General of the United States Army. Johnson, as Chernow puts it, intended his tour “to publicize his pro-southern policies, berate congressional Republicans, and woo Democrats in the fall elections.” Those policies could be summed up in Johnson’s declaration that “this is a country for white men, and by God, as long as I am President, it shall be a government for white men.” And, as President, he wanted Grant, who had led the Union forces to victory, to join his roadshow.

Grant had initially demurred but finally gave in after, as one of his aides wrote, Johnson grew so insistent that Grant felt “it would be indecorous any longer to object.” Johnson was his Commander-in-Chief, and Grant did not want to defy him, let alone be uncivil. And perhaps, his staff thought, it might do some good. One aide wrote that the trip might secure Grant’s place “in the confidence of Mr. Johnson, enabling him to fix up the Army as it should be.”

It didn’t work out that way. Johnson was angry that Grant got more applause than he did; the tour, Chernow writes, showcased “the coarse, vulgar side” of Johnson’s nature. The President yelled back at crowds that heckled him; he also openly expressed his racist views. When a man in Cleveland called out “hang Jeff Davis”—the President of the Confederacy, who was then under indictment for treason—Johnson shouted back with the names of two leading abolitionists: “Why not hang Thad Stevens and Wendell Phillips?”

“Is this dignified?” someone in the crowd asked.

“I care not for dignity,” Johnson replied.

Grant wrote to his wife that he saw the President’s speeches as “a National disgrace.” But he continued on the tour to Chicago and then back to Cincinnati, where, according to a newspaper report, he chastised a crowd of people expressing support for him and a rejection of Johnson, saying “I am no politician. The President of the United States is my Commander-in-Chief.” Finally, claiming to be ill, Grant headed back to Washington early. ...