Harry Truman, a former US Senator, served as Franklin Roosevelt's last vice-president, and became president upon Roosevelt's death. Some of FDR's advisors feared that the prior VP, Henry Wallace, was too far left, and they also thought that Roosevelt's death was imminent, so they sought a different nominee. FDR said he would accept either Truman or Supreme Court Justice William O. Douglas. The advisers settled on Truman. This passage from Wikipedia is informative:
"Truman's brief vice-presidency was relatively uneventful. Roosevelt rarely contacted him, even to inform him of major decisions; the President and Vice President met alone together only twice during their time together in office. He had rarely discussed world affairs or domestic politics with Roosevelt and 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. Truman had been vice president for only 82 days when President Roosevelt died on April 12, 1945. That afternoon, Truman...received an urgent message to go immediately to the White House. Truman assumed that President Roosevelt wanted to meet with him, but Eleanor Roosevelt informed him that her husband had died...Truman's first concern was for Mrs. Roosevelt. He asked if there was anything he could do for her, to which she replied, 'Is there anything we can do for you? You are the one in trouble now!'"
Good luck on this one! Be safe. Have fun!