Have Calvin Coolidge decide to run for another term in 1928. He would do absolutely nothing come 1929, and the results would be *far* worse. We're talking about no RFC, not even limited attempts to mitigate the Great Depression. The farm and labor unrest escalate, with Coolidge utilizing the military to put down strikes left and right.
Roosevelt wins the Democratic nomination and defeats Herbert Hoover (closer margin, because Hoover still has a reputation ITTL) but gets killed prior to becoming President in 1933. Thus John Nance Garner takes the oath of office on March 4, 1933. Garner does little to mitigate the Depression (think OTL Hoover), loses control of Congress to the Republicans again in 1934 (or at least loses the House, making him pretty impotent) and left parties start winning elections. Strikes go worse, military brought in again.
In 1936, the contest is between Garner, Republican nominee Herbert Hoover, and Frank Olsen, running as the candidate of the Farmer-Labor Party (which is endorsed by the Socialist Party). Hoover wins the election this time around, and enters the White House pledging to do more to combat the Depression.
It doesn't matter, though. Labor unrest has hit peak levels in the midwest, and farmers and workers alike have thrown their support behind the Farmer-Labor Party, which has allied itself with Wisconsin's Progressive Party and the Socialist Party led by Norman Thomas. The Communist Party, under the order of Stalin, supports the 'Popular Front' policy and decides not to run candidates against FL candidates. Farmer-Laborites and Democrats win big in the 1938 Congressional Elections, essentially emasculating the fledgling Hoover administration and demanding reform or (in the more radical circles) revolution. Many areas of the country are in perpetual martial law and have been for almost a decade -- as the war begins in Europe, Hoover pledges neutrality, while the Democrats pledge no policy and the Farmer-Laborites advocate pacifism.
In 1940, Hoover loses the White House to Huey Long, a Democrat endorsed by the Farmer-Labor Party with strong anti-business and anti-elite positions. Long quickly moves to reform the economy, introducing a number of policies to jump-start the economy (an easy task, owing to Democratic and Farmer-Labor majorities in Congress) and reform conditions for labor. The Japanese hit the Philippines in 1942, bringing America into the war on the side of the Allies. Long doesn't live to see the end of the war (although he does live to see the end of the war in Europe), being struck down by an assassin's bullet in 1945. The Vice President, Democratic-Farmer-Labor (a merger occurred in '44 at the behest of the President) leader Henry Wallace becomes President and prosecutes the endgame with Japan, resulting in a bloodbath on the home islands of Japan with the war finally coming to a close in 1946.
As America exits the war, hopes of a return to normalcy, an end to the crack down on dissent that happened under Long, the military rule under Coolidge, Garner, and Hoover, and the like, are put off. Wallace seeks to ally the United States with the Soviet Union before abruptly shifting course...it doesn't matter, however, because Bob Taft beats Wallace by a hefty margin in 1948 and proscribes a return to isolationism...