I actually feel like they don't realize they're lying. Leftists learn a "value system" that at its core is built on the foundation of lies; however, leftists don't think this (or know this). They believe the core of their moral values are in fact true.

What ends up happening is something in reality will clearly show that the value system of a leftist is false; however, in order to accept this conclusion the leftists would therefore have to challenge his foundational core value-system as a whole which can be daunting and very uncomfortable for people. Therefore, instead of challenging their beliefs when new information is given to them which would normally force people to have to accept a different perspective, they run all sorts of mental gymnastics to avoid having to accept that their value-system is based on false notions. In some cases, they "outright lie" but in their minds they aren't lying. In the leftist's mind, they are being true to their moral value system which they believe is true and although they are saying a lie, in their mind it is believed to be the truth because of these core foundational moral values being false to begin with but accepted as true.

Essentially, in some regards it is like mind-control and/or brainwashing. It's similar to how in the novel 1984, the "party" convinces the people that lies are truth and truth is lies. The people who worship the party believe whatever the party says is true even if it's clearly a lie. Leftists believe what they are saying, which is essentially given to them to believe by "the party" is true even when what they are saying is clearly a lie because leftists are this brainwashed and/or mind-controlled by their false moral-value system (the party).