If the Democrats win the House in November, they’ll come under pressure to impeach President Trump. Even if Robert Mueller fails to turn up some astounding surprise, many Democrats want to impeach Mr. Trump because they simply don’t like him. Since the Constitution specifies that a president can be impeached for “Treason, Bribery, or other high Crimes and Misdemeanors,” such a move would mean Democrats consider being disliked by the House majority to be a disqualifying crime. That is precisely what many members of Congress thought 150 years ago this week, at the conclusion of the first impeachment of a sitting president, Andrew Johnson. The 17th president’s impeachment offers the important lesson that although the mechanism for impeachment is easy, the subsequent process of trial, conviction and removal from office is not. A failure at that stage of the process covers everybody with embarrassment—impeachers and impeached alike.

The problem with impeachment began at the beginning, during the 1787 Constitutional Convention. Delegates were uneasy about the idea of concentrating all executive powers in the hands of the president. What, James Madison asked, was to protect “the Community against the incapacity, negligence or perfidy of the chief Magistrate”? But although they agreed the president must be checked, the delegates were unsure about which branch of government ought to bell the cat. Congress? The judiciary? That, objected South Carolina’s Charles Pinckney, would effectively destroy the president’s independence, while New York’s Rufus King suggested it would dump the principle that “the three great departments of Government should be separate and independent.”

The solution was to make “all civil Officers of the United States” liable to impeachment for “Treason, Bribery, or other high Crimes and Misdemeanors.” But the founders divided the process between the House, which would move the impeachment articles, and the Senate, which would conduct the trial to convict. When the subject was the president, the chief justice would preside. What’s more, the Senate would have to summon a supermajority of two-thirds for a conviction. It would take some serious provocation to make impeachment look worth the effort.

That, however, is just what Andrew Johnson provided. After Lincoln’s assassination, he inherited the presidency without any clear public mandate. The Civil War had just come to its exhausted close, and as a Southerner, Johnson imagined that the best solution was to let bygones be bygones and accept the South back into the Union as quickly as possible. His only requirement was that the Southern states eliminate slavery, the war’s principal cause.

That, however, wasn’t enough to suit Congress, where Lincoln’s Republican Party held ample majorities in both houses. They balked at Johnson’s easy-pass approach and began enacting legislation to assert control over the reconstruction process….

Americans prefer to choose their presidents with elections, and whenever impeachment is used in an attempt to nullify those choices, the results aren’t happy for anyone. That was true in 1868, and as both Andrew Johnson and his accusers might warn us, it remains true after a century and a half.