During the presidential campaign in 1864, the outcome of the Civil War was still very much in doubt, and while the Republicans renominated Lincoln for the presidency, they made an unprecedented move by nominating a Democrat, Andrew Johnson, for Vice President. Senator Johnson was notable for being the only Senator to remain loyal to the Union even when his state, Tennessee, had seceded. At the time, nominating Johnson seemed to be a mere token of goodwill - little did Republicans know what would happen the following April.

Today, Lincoln is almost universally regarded as one of the country's greatest presidents, while his successor is remembered for being the first president impeached by Congress. Indeed, he came perilously close to losing his office. On the surface, the story is a simple one: Johnson intentionally violated several laws, including the recently passed Tenure of Office Act, which was designed to limit his involvement with Reconstruction. However, Johnson's impeachment owed a great deal to his temperament, the unique situation Congress found itself in at the end of the Civil War, and the challenges facing the Republican Party now that slavery was abolished. For most of the republic's history, the executive branch accrued power at the expense of the legislative branch, and impeaching Johnson over his failure to abide by the Tenure of Office Act (itself an encroachment on the president's right to choose members of his cabinet) was a moment when that dynamic could have been reversed. Similarly, opposition to Johnson - and the ordeal of his trial in the Senate – helped forge the Republican Party into a cohesive political party that dominated the presidency for most of the next seven decades.