Fearless Leader
Donor
Well, if he does sign any civil rights bills during his term (and he seems to be the sort of Commander-in-Chief who wants to see his laws enforced; he won't tolerate Southern governors who defy any Federal laws), you could see African Americans voting generally Republican, while Jewish Americans generally stay Democratic.
Might Earl Warren work as Doug's running mate? He seems to be an OTL political figure who never gets much attention in alternate history.
Warren could very well balance MacArthur on the GOP ticket, however becoming President in his own right would be difficult. Joe Junior is definitely not in a place to run in 1952 like OTL and would most likely wait until 1960 to do so winning a first term in office.
I could also see internal communism becoming somewhat of a big deal in TTL's 1950's what with the end of a 24 year Democratic stranglehold on the WH. Alger Hiss and all that may come out later with substantially less effect but still, a general sense of paranoia.
As for civil rights I could definitely see MacArthur passing some legislation to encourage African Americans to vote in the South. How effective that would be...well that's another thing. Perhaps de-segregation comes to a more gradual end (unfortunately it's still going to be marred with considerable ammounts of violence but nowhere near OTL). It could also lead to the fracture of the New Deal Coalition with Southern Conservatives forming their own party...
It would also be interesting to see how MacArthur deals with Stalin's death and whoever replaces him. Beria came pretty close in OTL, perhaps he's a bit luckier in TTL? Other potential successors? Perhaps Zhukov and the army get tipped off?
Here's a tentative list of Presidents for a Potential TL
List of United States Presidents in "The Third Term..."
-Franklin Delano Roosevelt (1932-44)
-Cordell Hull (1944-1952)
-Douglas MacArthur (1952-1960)
-Joesph Kennedy Junior (1960-????)