Senator Harry S. Truman from Missouri was nominated to be President Franklin D. Roosevelt's running mate in his bid to be re-elected for a fourth term.

How the nomination went to Truman, who did not actively seek it, is, in the words of his biographer Robert H. Ferrell, "one of the great political stories of our century.

Truman's predecessor as vice president, the incumbent Henry A. Wallace, was unpopular with some of the leaders of the Democratic Party, who disliked his liberal politics and considered him unreliable and eccentric in general.

[2] By contrast, the Gallup poll said that 2% of those surveyed wanted then-Senator Truman to become vice president.

[3] To overcome this initial deficit, the leaders of the Democratic Party worked to influence the convention delegates, such that Truman received the nomination.

[4] A powerful group of party leaders tried to persuade Roosevelt to not keep Wallace as vice president.

Among the possible candidates were James F. Byrnes, Roosevelt's "assisting president", who initially was the prominent alternative, Associate Justice William O. Douglas, U.S.

Senators Alben W. Barkley and Harry S. Truman as well as the industrialist Henry J. Kaiser and Speaker of the House Sam Rayburn.

[5] Finally the group decided on Truman, but this decision was secondary to the goal of not nominating Wallace.

[7] In May, the president sent Wallace on a trip to China and the Soviet Union, probably with the intention to get him out of the country at an inconvenient time and to obstruct his campaign.

He supported the administration on most issues, was acceptable to the unions, and he had opposed Roosevelt's reelection to a third term, which pleased conservative anti-Roosevelt Democrats.

The next day Hannegan and Walker thus tried to convince Wallace and Byrnes to withdraw, but they refused unless the president himself asked them.

But Wallace nevertheless understood the president's real intentions, and he wrote in his diary, "He wanted to ditch me as noiselessly as possible."

They obtained a typewritten version of the note from July 11:[7] Dear Bob: You have written me about Harry Truman and Bill Douglas.

[16] Truman biographer Conrad Black wrote that Tully did switch the positions of the names, but it was probably at Roosevelt's wish.

[23] On Monday evening the party leaders telephoned Roosevelt, saying that labor would not accept Byrnes and mentioned Flynn's concern as well.

[29] One reason was that he had put his wife Bess on his Senate office payroll and he didn't want her name "drug over the front pages of the papers".

Roosevelt replied loudly, so everyone in the room could hear, "Well, tell him if he wants to break up the Democratic Party in the middle of a war, that's his responsibility," and slammed down the receiver.

[31] On Thursday, July 20, Hannegan released the letter which Roosevelt had given him on board the train, and its text appeared in the newspapers the next morning, but as it mentioned both Truman and Douglas it made people confused.

Both Ferrell and McCullough compare the way Truman was nominated with more recent presidential elections, where the candidates must participate in state primaries to receive delegates to the national convention.

"[38] Ferrell asks himself if Truman, who appeared to gain the office without the effort, in reality was playing a calculated and sly game.

Roosevelt disliked ambitious people, and Truman knew this, so it was probably an advantage to be humble and deny he was a candidate.

[39] As a border state senator and a political moderate compared with the liberal Wallace and the conservative Byrnes, Truman was humorously dubbed the "Missouri compromise."

[7] Few Americans wanted to change their leadership as the Second World War was still going on, so Roosevelt and Truman easily defeated the Republican nominee Thomas E. Dewey and his running mate John W. Bricker.

In the 2020 primaries, progressive Senator Bernie Sanders of Vermont was a top contender in a crowded field of candidates, but most of the candidates dropped out and endorsed former vice president Joe Biden of Delaware, giving Biden a decisive victory over Sanders.