This Real News Network segment covers an important and not well recognized bit of American history: how a hugely popular, progressive vice president who would otherwise have become president was sidelined by the Democratic party machinery.
You can read the transcript at Real News Network. The key bit is the excerpt from a letter Roosevelt wrote to the Democratic party in 1940 resigning over their opposition to Henry Wallace:
The Democratic Party has failed when it has fallen to the control of those who think in terms of dollars instead of human values. Until the Democratic Party shakes off all the shackles of control fastened upon it by the forces of conservatism, reaction, and appeasement, it will not continue its march to victory. The party cannot face in both directions at the same time. Therefore I decline the honor of the nomination for the presidency.