Well, all signs in character and outlook point to Democrat, but the fact that he was from upper New England point to Republican at the time the film was made.
What?
Watch the movie again. He is from Missouri.
Democrat, probably.
I thought he was from Tennessee.
It says Missouri in the movie, I'm pretty sure.
I'll have to watch the movie again, it's been such a long time since I've seen it. Anyway, I thought he would have to be a Democrat. He would almost have to be, being chosen by the governor to replace a Senator in Missouri in the 1930s. Also, there were a few clues in the movie that led me to believe that he was part of the majority party.