His first VP was John Garner, who was his VP for his first and second term. Then for his 3rd term, he swapped to Henry Wallace. Then for his 4th and 5th term, he swapped to Harry Truman, who became president after FDR died in office. Were these swaps based more on the VP's decisions or did FDR have an issue with Garner and Wallace that led to him choosing a different VP? Or did FDR's health impact his decision to choose a different VP, maybe thinking the idea of a VP becoming president was more likely as his health progressively got worse? I get that we can't know for sure what was going on in FDR's head at the time, but what was happening politically at the time and did FDR ever make a statement on the topic?
EDIT: he was elected 4 times, not 5. And Truman only was VP for one term, not two. M'bad
FDR switched VPs for a variety of political reasons, motivated by his own decisions, the desires of the VPs, and the mood of the party.
Let’s start with John Garner: FDR picked Garner (who was also a democratic candidate in 1932) as VP as part of a deal which allowed Roosevelt to win the nomination. At the time, you needed to win 2/3 of the delegates in order to secure the nomination. Roosevelt, who had the support of the majority of delegates but not the requisite 2/3, picked Garner as his VP in order to have Garner’s delegates in his column and win the nomination. In 1936, the DNC (at Roosevelt’s insistence) changed the convention rules to allow a candidate to need only a majority of delegate support in order to secure the nomination. Garner, a Southern conservative Democrat, and Roosevelt, a liberal Democrat, had a massive political falling out during Roosevelt’s second term due to policy disagreements, so massive that Garner could be considered the cause of the legislative failures of Roosevelt’s second term.
Then comes 1940. Given that Roosevelt had already served 2 terms, it was initially presumed that he would follow the 2-term tradition and stand down. While many called for him to run again who felt that FDR was the only Democrat who could win (for comparison, remember how Democrats felt about Obama in 2016), Roosevelt privately told other potential contenders that he wouldn’t run and that they should feel free to run. Garner was considered the front runner in the event that FDR didn’t run but Garner claimed that he and Roosevelt had both agreed to had retire in 1940 after 2 terms. However as it became clearer that Roosevelt was making himself available to run, Garner decided to enter the race as the candidate for democrats opposed to a third Roosevelt term. Lo and behold, Roosevelt gets “drafted” at the convention and wins the nomination with 86% of the vote. Garner was philosophically opposed to a president serving a third term so he refused to be on the ticket.
So Roosevelt picked Henry Wallace, who was closely aligned with FDR on domestic and foreign policy. While many in the party felt that Wallace was too liberal of a pick, Roosevelt indicated that if Wallace wasn’t the VP nominee then FDR would refuse the nomination. Given the problems that Garner had caused him, Roosevelt wanted a VP who had neither the desire nor the ability to challenge him politically. Since FDR had abolished the erstwhile 2/3 requirement, he didn’t need to worry about losing the nomination and could make such a demand. After lobbying efforts from Eleanor, the party agreed to accept Wallace as VP.
Fast forward to 1944. Given that the US was in the midst of a war, it was expected that Roosevelt would run for a fourth term. However there were questions as to whether Roosevelt was healthy enough to serve a full term and a belief that the VP nominee would become the 33rd President. Therefore, the concerns about Wallace that were voiced in 1940 resurfaced. Roosevelt was more accommodating this time and agreed to pick a different VP nominee.
Why he was more accommodating and agreed to drop Wallace is unclear. While FDR’s doctors knew how precarious Roosevelt’s health was, FDR was in the dark about it (or at least in denial about how sick he was). Perhaps, being the political fox that he was, FDR knew that the anti-Wallace sentiment would grow and he wanted to avoid the embarrassment of being on the losing side. There are some reports that Roosevelt didn’t plan to serve a full fourth term and would’ve resigned after the war ended (which he knew would likely be soon) so he knew that the VP would become President. For whatever the reason, FDR went along with it and agreed to drop Wallace. However the switch from Wallace to Truman was not related to any disagreement between the President & VP and Roosevelt, who felt the party had robbed Wallace, made amends with Wallace by offering him any cabinet post except for Secretary of State. Wallace picked the Commerce Department because that Secretary was expected to play a key role in the transition to a postwar economy. Also, FDR agreeing to nominate Wallace to the post is a sign of the high regard he continued to hold for Wallace