In May 1868 the Republicans, as everyone expected, nominated Grant for president on a platform that called for black suffrage in the South but discreetly avoided proposing it nationwide. Johnson hoped for and sought the Democratic nomination, believing that his fight against congressional Reconstruction entitled him to it. Most Democrats now considered him, rightly enough, a political liability, and at their convention in July nominated Horatio Seymour, a former New York governor. Although disappointed, Johnson—out of hatred for Grant, if nothing else—did what he could during the campaign to help Seymour. Grant won by 214 electoral votes to Seymour's 80, but his popular majority was only 300,000, and even that was the result of the enfranchisement of blacks and the disfranchisement of whites in the South. Hence, early in 1869, the Republicans placed black suffrage on a permanent foundation by ramming through the Fifteenth Amendment, which stated that no citizen could be deprived of the vote for reasons of "race, color, or previous condition of servitude." This turned out to be the final major measure of congressional Reconstruction, and like the first, it became law over Johnson's futile protests.
Johnson finished his few remaining months as president in relative peace. At noon on 4 March 1869 he left the White House without, as is customary, attending the inauguration of his successor. He returned to Tennessee and soon plunged into politics, making several unsuccessful attempts to secure a seat in Congress. Finally, in January 1875, the Tennessee legislature, now controlled by Democrats, elected him to the United States Senate. On 5 March he took his seat in the Senate, thereby becoming the only former president to serve in that body. He did not serve long. On 28 July, while visiting one of his daughters in Tennessee, he suffered a stroke and three days later died. He was buried near his Greeneville home, his body wrapped in the United States flag and his well-thumbed copy of the Constitution beneath his head.