It started with the Weather Underground and Socialist Democratic Students (SDS) groups in the 1960’s. They incited violent riots at atheist 1968 Democrat National Convention, but their actions did not achieve their goals. The Weather Underground split in two, with one leader believing success would come not through violence, but by working within the system. Neither the Communist or Socialist parties, being pariahs at the time, were viable choices. Thus the infiltration of the Democrat party. Hillary wrote her thesis (presently sealed so nobody knows what she wrote) at Columbia University on Saul Alinsky, a radical Marxist.
You should be able to find more if you look.