One possibility is to have Harry Stassen win the Republican nomination, and then win the Presidency in 1952. Afterwards he then goes on to ban the Communist Party, which has the effect of driving many Communists underground. As the Red Scare intensifies, the paranoia of potential "secret Communists" and grossly overestimating their influence adds even more fuel to the fire, in much the same way that paranoia surrounding reactionary conspirators and wreckers enflamed the Soviet purges. Throw in right-wing politicians and business interests more successfully using the scare as a means of attacking progressive rivals and unions, and with the various security and intelligence agencies successfully using the spectre of Communism to lobby for increased powers and funding. Finally prevent McCarthy from getting censured (which in turn would likely prevent his alcoholism and get him to live longer). If possible find some way to get the left-wing of Democratic Party to split off completely and merge with various other non-Communist left-wing groups. The potential power bloc that this off-shoot would form would be seen as a threat by the major parties, encouraging moderate Republicans and Democrats to close ranks with their more right-wing and red-baitie compatriots, whist also providing a source of potential "secret Communist infiltrators" (especially if Henry Wallace is still kicking around), who are only getting dragged before the HUAC for national security reasons, and not because they're running in elections against established politicians.