structural:reformatting, structural:expansion
Vice presidency (1945)

Roosevelt's advisors knew that Roosevelt might not live out a fourth term and that his vice president would very likely become the next president. Henry Wallace had served as Roosevelt's vice president for four years and was popular on the left, but he was viewed as too far to the left and too friendly to labor for some of Roosevelt's advisers. The President and several of his confidantes wanted to replace Wallace with someone more acceptable to Democratic Party leaders. Outgoing Democratic National Committee chairman Frank C. Walker, incoming chairman Hannegan, party treasurer Edwin W. Pauley, Bronx party boss Ed Flynn, Chicago Mayor Edward Joseph Kelly, and lobbyist George E. Allen all wanted to keep Wallace off the ticket. Roosevelt told party leaders that he would accept either Truman or Supreme Court Justice William O. Douglas.

State and city party leaders strongly preferred Truman, and Roosevelt agreed. Truman had repeatedly said that he was not in the race and that he did not want the vice presidency, and he remained reluctant. One reason was that his wife and sister Mary Jane were both on his Senate staff payroll, and he feared negative publicity. Truman did not campaign for the vice-presidential spot, though he welcomed the attention as evidence that he had become more than the "Senator from Pendergast". Truman's nomination was dubbed the "Second Missouri Compromise " and was well received. The Roosevelt–Truman ticket achieved a 432–99 electoral-vote victory in the election, defeating the Republican ticket of Governor Thomas E. Dewey of New York and running mate Governor John Bricker of Ohio. Truman was sworn in as vice president on January 20, 1945. After the inauguration, Truman called his mother, who instructed him, "Now you behave yourself."

Truman's brief vice-presidency was relatively uneventful. Truman mostly presided over the Senate and attended parties and receptions. He kept the same offices from his Senate years, mostly only using the Vice President's official office in the Capitol to greet visitors. Truman was the first vice president to have a Secret Service agent assigned to him. Truman envisioned the office as a liaison between the Senate and the president. On April 10, 1945, Truman cast his only tie-breaking vote as president of the Senate, against a Robert A. Taft amendment that would have blocked the postwar delivery of Lend-Lease Act items contracted for during the war. Roosevelt rarely contacted him, even to inform him of major decisions; the president and vice president met alone together only twice during their time in office.

In one of his first acts as vice president, Truman created some controversy when he attended the disgraced Pendergast's funeral. He brushed aside the criticism, saying simply, "He was always my friend and I have always been his." He had rarely discussed world affairs or domestic politics with Roosevelt; he was uninformed about major initiatives relating to the war and the top-secret Manhattan Project, which was about to test the world's first atomic bomb. In an event that generated negative publicity for Truman, he was photographed with actress Lauren Bacall sitting atop the piano at the National Press Club as he played for soldiers.

Truman had been vice president for 82 days when President Roosevelt died on April 12, 1945. Truman, presiding over the Senate, as usual, had just adjourned the session for the day and was preparing to have a drink in House Speaker Sam Rayburn 's office when he received an urgent message to go immediately to the White House, where Eleanor Roosevelt told him that her husband had died after a massive cerebral hemorrhage. Truman asked her if there was anything he could do for her; she replied, "Is there anything we can do for you ? For you are the one in trouble now!" He was sworn in as president at 7:09 p.m. in the West Wing of the White House, by Chief Justice Harlan F. Stone.