Marxism has its origins also, so there can be forms of Leftism from a common ancestor - Hegel being one of them, Kant being another. Leftist academics who studied Marx digested it and then passed it on to their students as progressivism.
Marx consolidated what later became what the Left holds dear.
Many Leftists, for example, may not have reasoned it out, but give evidence in their thought that they believe in the Labor Theory of Value. That holds that rather than supply-and-demand, the value of a product is based upon how much labor was put into it. So a chair that took 4 hours to build is worth twice as much as one that took 2 hours — even if they are exactly the same in every way. Absurd, I know, but it guides many Leftist policies.
My point is that leftism is considerably older than Marxism, which is one of its branches, though dominant.
For example, many conservatives claim fascism and Nazism are socialist and leftist. I don’t entirely agree, as I think they’re a weird mixture of leftism and (European variety) blood and soil right-wingery.
But you can’t have a theory where all leftism is Marxism and at the same time claim that fascism is also leftism, since fascism is inherently opposed to Marxism.
Here’s decent article about the many variants.
http://www.philosophybasics.com/branch_socialism.html#Types
Liberals deny human nature and project a fake utopia that can only be implemented by force, not economics or belief in the human being.
Their fantasies of equality only work when there is a police state that moves God to the State and all power to all-knowing party members/politicians and bureaucrats.
In the end, the human desire to work hard for their families erodes as the state take what it wants and redistributes it to preferred people.
It is absolute and utter tyranny, and the way to seal off freedom for the enslaved. Barbed wire, monitored internet, radio, TV and bread lines will eventually result.
Everyone here in FR will have to decide whether to live on crumbs and survive, barely, or fight for freedom. It’s coming, like the sound of a tidal wave in the distance, getting louder by the second.