For this to work, you have to have a perfect storm in the United States.
(1) Isolationism returns in the postwar United States. This means that either Henry Wallace becomes POTUS on April 12, 1945, or Truman does, and then loses re-election to Bob Taft in 1948. Regardless of what happens, you have no Marshall Plan and no NATO.
(2) As a result, Western Europe falls to the Soviet Union. Germany is united as the German Democratic Republic, Italian elections in 1948 bring the Communists to power, leaving only Britain and France standing against the Soviet threat. By 1968, however, a student and workers revolt in France establishes the People's Republic of France. The United Kingdom is alone in a sea of 'Red' European regimes, as Portugal's Carnation Revolution leads to the installation of a communist government there as well. Spain and Britain, along with the Nordic countries, hold out and form an anti-Communist alliance, though there's no telling how long it will actually hold up in the face of expanding Communist influence and a lack of interest on the part of the United States.
(3) In Asia, the PRC invades and deposes the ROC on Taiwan, uniting China fully under the red banner. North Korea defeats South Korea and establishes a united DPRK, and Japan, which has been split like Germany was after the war, is governed by a communist North and a authoritarian South. The North eventually defeats the South following a general war, repeating patterns seen in Korea and in Viet Nam. India aligns with the Soviet Union openly, as does Israel, as a buffer against Arab aggression.
(4) African colonial regimes are largely replaced by Soviet satellite regimes. In South Africa, a bloody uprising deposes Apartheid in the early nineties, installing instead a communist state there.
(5) The rest of Europe finds itself under siege as well. Labor unrest in Britain leads to a general strike and a worker's revolution in the seventies, deposing the Queen and installing the People's Republic of Britain. With the death of Franco, a second Spanish Civil War occurs, with communist forces succeeding outright, establishing the Third Spanish Republic.
(6) Latin America quickly becomes a hotbed of communism as pro-American regimes are deposed by communist regimes. Cuba and Chile are especially of note here, as is the People's Republic of Brazil. Economic unrest in Mexico leads to a peasant revolt in the late eighties which will eventually topple the Mexican government in the nineties, leading to the establishment of the 'People's Republic of Mexico' under Subcomandante Marcos.
(7) As a result of all of these changes, the United States has, by 1989, become a paranoid fascist state. Student unrest in the late sixties lead to draconian anti-communist laws being passed and eventually, most Americans just gave up on waiting for elections that never came. However, a general strike in 1989, provoked by American communists, did shake the existing government enough to begin reforms. By 1991 however, it was clear that reforms would not work and the nation split apart, with various communist regimes replacing whatever was left of the United States by the early 2000s.