You had a Communist sympathizer who visited the Soviet gulags and did not speak out as VP for FDR.
Liberalism did not change with the death of JFK. Socialists felt victorious after they hounded McCarthy. The KGB files showed that the “Red Scare” wasn’t a lie.
Meanwhile we have a Global Warming Scare today and Democrats like RFK Jr. calling anyone who denies it a “traitor” and says that we should be tried for “treason” to the United States.
A lot of far-left extremists have been in the Democrat Party for a long time, I agree. But I think that the rank-and-file voters did not knowingly give leadership positions to far-left extremists until the 1960's.
I think that Moscow, after WWII, tried to sway the US political system. McCarthy and Nixon helped slow that down in the 1950's. But I think the assassination of so many Liberal heroes in the 1960's, combined with behind-the-scene help from Russia and China, finally succeeded in changing the nature of the Democrats into a party where you have to be crazy to adhere to their view of the world.