Johnson served as a United States Senator from Tennessee at the time of the at the beginning of the American Civil War. He was the only Southern Senator not to quit his post upon secession, and became the most prominent War Democrat from the South. In 1862 President Lincoln appointed Johnson military governor of Tennessee, where he proved energetic and effective in fighting the rebellion. Johnson was nominated for the Vice President slot in 1864 on the National Union Party ticket, displacing incumbent Vice President Hannibal Hamlin, with Lincoln running for re-election.
When Abraham Lincoln was killed by a sniper on July 12, 1864 while inspecting the ramparts at Fort Stevens north of Washington, DC, Hannibal Hamlin became president, and Andrew Johnson was sidelined. Shortly after Lincoln's death, Johnson could do nothing but glare up at Hamlin from the audience as Hamlin was inaugurated.