Factually, Marxists have been infiltrating and organizing in the US since just before the turn of the Twentieth Century.
It increased in WWI and again 1921. Lenin ran several philosophical indoctrination programs.
Joe Stalin quickly instituted and funded hundreds of espionage, corruption, and sabotage programs throughout the US (and the rest of the free world) and most of those efforts became self sustaining and have never stopped.
A prime example of a Stalin created and funded program is the ACLU.
In the past thirty years, they seem to be multiplying like bacteria on fresh feces. Sure, they changed their name to Progressives but that didn’t slow them down. I will say they are much bolder than when I first started suspecting something was amiss.