Nobody stops Claude Pepper from getting to the podium in Chicago in 1944. As a result, Wallace is nominated, succeeds when FDR dies, and names Pepper as his VP. Wallace is popular among the farmers of the South given his agricultural history. Yeah, Truman was a farmer too. Both of them embraced civil rights. Nope.
Ah, yeah, President Dewey in 1948 is a good way to keep southern whites in the New Deal Coalition. Let's go a POD earlier: Lyndon Johnson wins the Senate seat in 1941, becomes Senate Majority Leader in 1944. Lyndon is tapped as FDR's VP, but Wallace wins a surprise convention fight (as he nearly did IOTL). By 1952, Dewey is bogged down in Korea and loses to Landslide Lyndon. LBJ is able to pass Civil Rights successfully (Dewey tried, since it divided the Democrats), but not the Great Society. Dewey is remembered as one of the most unpopular presidents in history, and control of the party falls to Robert Taft (briefly) and then John W. Bricker. Richard Nixon and Joe McCarthy are kicked out of an anti-Dewey Congress, Nixon by Democratic hawk Ronald Reagan. Recently-elected Governor Nelson Rockefeller swoops in 1956 to take the nomination from frontrunner Bricker, and loses to Johnson in another landslide. Bricker runs for President three times, in 1960, 1964, and 1972. By the 1980s, he occupies a similar place in the GOP than William Jennings Bryan did in the Democratic Party in the 1910s.