I find fascinating the shenanigans, according to Oliver Stone's - The Secret History Of the United State (on Showtime) - which occurred at the 1944 Democratic Convention. The successful attempt to prevent Henry Wallace from getting the Vice President slot.
Stone account was done with great drama. This was just beautiful TV!
The powerful feared Wallace because Wallace acted and talked like a communist. However Wallace had lots of grassroots support and would have succeeded if extensive underhand methods were not used against him.
Truman got the slot.