Liberalism is a political philosophy which generally embraces republicanism, constitutionalism, rule of law, political equality, free markets, private property, among other things. Setting aside the outright Fascists, the majority of American Conservatives are in-fact Liberals, though they have exceedingly terrible opinions about social equality. The spectrum of acceptable political discourse in the US is simply so narrow that it runs from Liberal to Liberal but enthusiastically bigoted.

From the Marxist perspective, culture, politics, and ideology are all outgrowths of the underlying material conditions. In the grand durée of history, the emergence of Liberalism coincides with the wave of Bourgeois revolutions which marked the transition from Feudalism to Capitalism as the prevailing mode of production (which didn't happen everywhere all at the same time). To this day it is the prevailing ideological framework under the conditions of Capitalism (except under conditions of overwhelming crisis where all pretenses of republicanism and rule of law are dropped to defend private property against the threat of social unrest).

The Marxist critique of Liberalism comes from a few angles. Naturally, we take issue with the inviolable right to private property. We believe in the collectivization of the means of production. Likewise, we take issue with the sort of free market absolutism which puts a price tag on everything under the Sun and turns everything into a commodity. We think the rule of law, republicanism, and constitutionalism are all for the birds insofar as these institutions stand as a bulwark against the realization social justice (which they invariably do under a Capitalist oligarchy).

But more fundamentally, we are Materialists. This gets really abstract, but basically there are two overarching schools of philosophical thought: Idealism and Materialism. Idealists believe that ideas are the driving force in nature, which the world is shaped by, while Materialists believe that our ideas are simply a reflection of the material world we find ourselves in. Liberalism is an idealist philosophy. It essentially proposes "This is the way things ought to be. If enough of us share this belief, it shall be so." Marxism is a materialist philosophy. It begins with an empirical study of the material conditions throughout history, attempts to identify the processes which are unfolding, the forces which transform society, and uses that as a basis for identifying the most fertile points of social struggle.