In a general sense liberalism involves a spirit of rebellion against God, promoting humanism and naturalism et al.
The core of liberalism is Marxism. Bella Dodd wrote in School of Darkness that CPUSA members were active in Liberal Party functions for the purpose of rooting out any thinking which conflicted with Marxism.
The Democrat Party of the early to mid-1900's was controlled by many of those same people.
To understand what liberalism is requires familiarization with the encyclicals on government as well as an understanding of American and world history. Sadly, most Americans hold Americanist views (we are 100% right in all we do and our enemies are 100% wrong, 100% of the time). That is in itself a Marxist strategy of propaganda. People do not know what a liberal is because they do not know what the Catholic teaching is that diametrically opposes it.
Nor do they understand conservatism as it has so often been redefined.