Of course his father, Henry C. Wallace, had been Secretary of Agriculture for a Republican President (Harding--continued for a while under Coolidge).
Your point is well taken. There was much opposition to Wallace from within the Democrat Party.
What I meant to say (and didn’t explain myself too well) is that Wallace did have a chance to secure the 1944 VP nomination and didn’t act upon it. The book “American Dreamer” by John Culver explains what happened at the ‘44 National Convention.
Roosevelt, in typical fashion, hemmed and hawed to pro and anti-Wallace factions as to whether Henry Wallace would be on the ticket again in ‘44. His comments led both sides to assume that either (a) Wallace would be removed from the ticket or (b) Wallace would be the VP nominee.
Rexford Tugwell said it best about Roosevelt “He had a trick of seeming to listen, and to agree or to differ partly and pleasantly, which was flattering. This was more highly developed as he progressed in his career and it was responsible for some misunderstanding. Finally no one could tell what he was thinking, to say nothing of what he was feeling.”