Communist ideology was spread very rapidly during the 30s and during the cold war and many major figures had ties to Moscow, sometimes directly, usually through close relationships with organisations with links to the soviets. As a result liberalism from the 30s onward believe the same tenets.

1: The belief marx-derived ideologies as the only solution to problems target populations feel strongly about and pinning political opponents as aides to crony capitalists, especially those which compete with them for political support from target populations.
2: Extensive use of marxist arguments, without stating the origin of their reasonning.
3: Since most of their assertions are incorrect, manipulation of people's emotions, demagoguery and extremism are common. While this is not unusual in politics it tends to permeate target groups quite strongly, young adults for example are targetted to the extent that there are now liberal arts colleges in the US that teach an exclusively "progressive" agenda.