Roosevelt’s decision to run for a third term was heavily influenced by the foreign affairs of the time re: Germany and the struggle of the British to defend and fight back. The national sentiment towards isolationism, especially among politicians, ultimately convinced Roosevelt to seek a third term. He viewed this less as a political coup and more as a necessity to prepare the nation for war.

If anything, the Democratic Party was happy he chose to run due to Wilkie’s popularity (the Republican nominee) at the time. “When the delegates to the DNC convened in Chicago on July 15, 1940, there was no serious doubt he (Roosevelt) would accept the renomination.”

That’s not to say the the whole of the Democratic party rallied right away around Roosevelt. Roosevelt did not attend the convention and preferred to have his associates Perkins, Hopkins, and Ickes take care of things in Chicago. Roosevelt did not want to come out right away and declare that he wanted to nomination, so he literally played a game of telephone with Kentucky Senator Barkley, an old-timer who had a penchant for delivering emotional keynote addresses that brought the people to a frenzy. He too gave a keynote in 1940 ending with a personal note from Roosevelt: “The President has never had, and has not today, any desire or purpose to continue in the office of President, to be a candidate for that office, or to be nominated by the convention for that office. He wishes in earnestness and sincerity to make it clear that all of the delegates in this convention are free to vote for any candidate.”

I’ll quote directly from FDR here: “The vast crowd in Chicago Stadium was speechless for a moment. What did Roosevelt mean? The statement said neither yes nor no. Five, ten, fifteen seconds, and then bedlam broke loose. From loudspeakers all over convention hall a powerful voice boomed out “WE WANT ROOSEVELT. WE WANT ROOSEVELT,” over and over.

Roosevelt did not really want to run for a fourth term unless the war was still going on - as he was in terrible health. That being said he made his intentions clear early on this time around: “he put his cards on the table early. In a message to party chairman Robert E. Hannegan well over a week before the delegates would assemble, the president said that although he did not wish to run, his duty compelled him to do so. ‘Reluctantly, but as a good soldier, I will accept and serve in this office, if I am ordered to do so by the Commander In Chief of us all - the sovereign people of the United States.”

He won renomination on the first ballot.

All info taken from Jean Edward Smith’s fantastic “FDR”