America could be a political pariah if slavery wasn't abolished or jim crow continued and or was more influential. combine that with imperialist behavior and a more popular soviet union. it wont lessen the power that the united states wields, but it would lessen the number of allies that it has for staging.
make it end of WW II, minorities return from war, the 50's go on .. MLK and X and others are killed, JFK killed, civil rights gets the hard hand. America turns more and more authoritarian to control its minority populations in response. this will sap the economy and of which one would expect a much more xenophobic America with much tighter boarder and immigration policies.
just for giggles and I know its unknowable, but have AIDS start during or following the war. infecting poorer regions and groups, this would also keep the old school white fear at high level.
So essentially The East is Red?
PoD is Stalin helps China with resources, winning the PRC the Korean War. Due to Soviet airpower, Chinese military strength,
and good diplomacy, the war is resolved without nukes being used outside of Korea. (Which failed, with the only real casualty of note being Mao Anying) Taiwan is surrendered in exchange for the 49th Parallel in Korea.
As such, McCarthy wins the election. The Vietnam War kicks off shortly after. McCarthy responds with nuclear holocaust, glassing northern Vietnam, and attempting to glass Southern China. He is stopped by his government, who aren't mass murderers.
Repeated defeats and brutality have weakened American resolve, and the PoWs from China come home. A fair amount of them have been radicalized by torture, and are causing a lot of problems. On top of this, American repression in Cuba is hurting its image. This results in fewer nations working with them in SA,and several nations turn communist, nationalizing key companies to the American economy.
Between radicalized veterans, economic depression, and steadily worsening race relations, the USA slips into chaos. Red Guards and Nazis clash in the streets. Trials have become shams. Frustrated citizens form paramilitary, and presidential candidate are turning as bad as FaT. American power is at a global low, and her troubles have made her into a pariah state.
TL;DR: Massive Chinawank makes USA into despised shithole.