Johnson vetoed their bills, and congressional Republicans overrode him, setting a pattern for the remainder of his presidency.Frustrated by Johnson's actions, Congress proposed the Fourteenth Amendment to the states, and the amendment was ratified in 1868. As the conflict between the branches of government grew, Congress passed the Tenure of Office Act, restricting Johnson's ability to fire Cabinet officials. Johnson had served as a Democrat in various offices prior to the Civil War, and he became one of the most prominent Southern Unionists after the start of the war. During the 1864 presidential election, the Republican ticket campaigned as the National Union ticket, and the National Union convention chose Johnson as the party's vice presidential nominee in large part because of Johnson's status as a prominent Southern War Democrat. This refusal prompted Congressman Thaddeus Stevens to introduce legislation to dissolve the Southern state governments and reconstitute them into five military districts, under martial law. State governments would be reformed after holding constitutional conventions.