Brenda Wineapple is the author of “The Impeachers: The Trial of Andrew Johnson and the Dream of a Just Nation.”

Only two men before Chief Justice John G. Roberts Jr. have presided over the impeachment trial of a president: William H. Rehnquist in the 1999 trial of Bill Clinton, and Salmon P. Chase in the 1868 trial of Andrew Johnson. But Roberts would be wise not to use Chase as his guide. If the Framers selected the chief justice for this role because they expected he would rise above the political fray, Chase was precisely the opposite of what they had in mind.

Chase, a leading abolitionist, had served as President Abraham Lincoln’s treasury secretary; before that, he was a senator, governor and presidential contender, challenging Lincoln for the Republican nomination in 1860. Four years later, allowing ambition to get the better of him, Chase covertly plotted to challenge Lincoln again. When the scheme was discovered, Chase denied involvement and supported the president. For Chase was an expedient man. So was Lincoln, who nominated Chase to replace conservative Chief Justice Roger Taney in 1864, since Chase was the best person to protect emancipation in the courts.

But Chase did not park his political ambitions at the bench. On the court during the Reconstruction era, Chase fiercely opposed what he called arbitrary military governments and military trials for civilians in peacetime. That made him attractive to moderate Republicans, he hoped, or, if need be, to states’ rights Democrats: He’d represent constitutional law, not military power, if he were to run for president in 1868.

That was the plan. As the Johnson impeachment opened, it was an open secret that the chief justice who would preside still wanted very much to be president and that he was no fan of the impeachment. One reason Chase bucked the impeachers was the absence of a vice president; if Johnson were removed from office, Chase’s enemy, the radical Benjamin Wade, president pro tempore of the Senate, was next in line for the White House.

So Chase did not disappear into the woodwork during the trial. As it began, he campaigned to organize the Senate as a legal body. That basically meant that the trial would be conducted mostly as though it were a legal proceeding, which slanted the definition of an impeachable offense toward an actual breach of law and away from broader questions of abuse of power or obstruction of justice.

This maneuver provided Johnson’s lawyers with ammunition: Did the president really break a law, or was he merely testing its constitutionality? And did the managers really prove their case beyond a reasonable doubt — “reasonable doubt” being a legal concept, not a legislative one.

It also meant that Chase, as presiding judge, sought the same powers any court judge would have; he wanted to decide legal disputes and rule on admissibility of evidence. The members of the House of Representatives prosecuting the president protested. “You are a law unto yourselves,” one of the managers addressed the senators, “bound only by the natural principles of equality and justice.” Chase said, then, in the case of a tie, he wanted to be able to cast a vote, exercising the same power that a vice president, as presiding officer of the Senate, would have.

After long debate, the Senate did concede much of its authority. Chase could decide on the admissibility of evidence, but any senator could call for a vote on his rulings. And the chief justice was allowed to cast tiebreaking votes on two procedural questions. If the managers disagreed with a point of law decided by the chief justice, only a senator — not the House managers — could appeal the decision.

Future president James A. Garfield, then a representative from Ohio, was disgusted by Chase’s maneuvering. “All the weight of his office and his influence were brought to bear to save Johnson,” Garfield reported. Several moderate Republicans were spotted dining with Chase during the trial or riding in his carriage. Energized, Chase’s backers raised money for his presidential bid while spreading rumors about the drinking habits of Gen. Ulysses S. Grant, who was the Republican favorite for the ticket. Later, it was alleged that money raised for Chase doubled as a fund for Johnson’s acquittal.

A few months after the trial, Frederick Douglass described Chase in 1868 as a man so “greedy for the presidency” he was willing “to disregard the sentiment of reserve which befits his present high office.” A few years later, poet Walt Whitman would be more concise: Chase was, he said, a “bad egg.”

As the Trump trial proceeds and both sides invoke past practice, those who want Roberts to take an active role should keep in mind that the Johnson impeachment may not offer the best guidance — and Chase certainly not the ideal role model.

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