Al Smith is narrowly elected in 1928, the Great Depression occurs under his watch with a presidency similar to Hoover's otl, and Theodore Roosevelt, Jr. is the Republican candidate in 1932.
Roosevelt offers a New Deal-style series of programs, creating a reverse New Deal coalition. The GOP remains the party of civil rights, and you have massive infrastructure programs like otl TVA and the like. He remains even more hawkish than his otl cousin's presidency, starting up the military industry in the South as early as the mid-30's when war begins to break out in Europe. He secures factory jobs for poor whites and civil rights for blacks, industrializes the south, with Republicans enjoying the massive supermajorities that the Dems enjoyed throughout the 30's in otl.
Perhaps an Eisenhower-style Interstate Highway System in the late 30's or early 40's. Conscription (and perhaps lend-lease) begins earlier, freeing up jobs formerly occupied by blue collar whites to blacks and women. Unemployment falls to nearly pre-Depression levels and with the massive capital investment, the postwar economic boom is even bigger. Then either MacArthur or Eisenhower is his successor; earliest Dem victory is probably 52 or 56, maybe with Joseph P. Kennedy Sr. advocating a "return to normalcy."
A Republican-wank scenario to be sure, but without needing to maintain the Southern Democrats' support like FDR did, civil rights (at least similar to the otl Civil Rights Act of 57 and perhaps earlier desegregation of the military, earlier desegregation of public schools, etc.) will advance much quicker than otl. We might even see the first black southern reps since Reconstruction by the late 40s and early 50s (most likely Republicans).
Again, this might be a bit over the top; this is definitely meant to be best-case scenario for the GOP
Moreover, there would probably be a conservative backlash in the mid 50s or early 60s.