1972: Agnew is indicted, Watergate goes hot, and the story about Nixon sabotaging peace talks in 1968 breaks, all before the election. The effect is an 11% swing in the popular vote, which just barely elects McGovern.
Now I'm going to make some wild assertions about McGovern as President which are probably questionable, but not IMO ridiculous. His election slogan was "Come home, America." I take this as a call for leftist neo-isolationism.
Taking office, he announces that the US will no longer oppose wars of liberation or support dictators.
McGovern starts by immediately and unconditionally withdrawing all US troops from Viet Nam and ending all aid to the Republic of Vietnam.
He also withdraws US forces from Korea in protest of the authoritarian regime of Park Chung-hee, and from the Philippines because of Marcos.
US forces also leave Okinawa to appease Japanese sentiment.
It's way too soon for the virulent left-wing anti-Israel sentiment common today, but there's some, and a higher-up in the McGovern administration is exercised over the Liberty incident, and his influence further poisons relations. During the Yom Kippur War, the US does not resupply Israel as OTL, and the emboldened USSR gives the Arabs enough additional help to win. (The US helps evacuate Jewish survivors at the end.)
(What other allies can the US abandon with effect? The Shah? El Salvador?)
Anyway, the Republic of China is next on the list. The US explicitly disclaims any security guarantee for the dictatorial Chiang Kai-shek regime.
With this de facto green light, and US forces withdrawn from the entire region, and the US clearly unwilling to intervene when not directly attacked, the PRC goes ahead with invasion of Taiwan.