A place to start would be to talk about the incredible violence the system doles out to the poor with full legal authority.

• Eviction / homelessness: is it not authoritarian to have half the country one missed paycheck away from armed thugs showing up and violently throwing them out to live on the street, where they will be vehemently despised by all of society?
• Healthcare: is it not authoritarian to tell people with fixable terminal illnesses, "you don't have a good enough job that provides insurance. instead of treating you, we will allow you to die,"?
• Prisons: is imprisonment not the height of authoritarianism? What about slavery? The school to prison pipeline, private prison bed quotas, and over policing and over sentencing of black communities are all legal means of ensuring and encouraging innocent people end up in prison, in actual slavery.

A liberal may argue that they don't like any of those things and hope to change them through reform, at which point you have to sell them on the idea of the dictatorship of capital; that they will not be able to reform anything until they are allowed to by the ruling class. I like the way @Tankiedesantski@hexbear.net describes it in his comment in this thread