It’s a topic that’s maybe a bit too dense and broad to reduce to a single short comment, but trying to simplify things a bit:

• Leftism is quite a nebulous term, its boundaries are delimited differently depending on who you ask. IMO It could be characterized as opposition to the capitalist economic framework (stemming from the question “Can the system be reformed?”, only answers starting with a “No” would be considered leftist). One of the main indicators of something being leftism lies in its adherence to the marxist principle of the working class being the owner of the means of production (or more famously, “seizing the means of production”).
  This point in itself would mean democratic socialists (demsocs) are considered leftists but social democrats (socdems) are not. I’m sure lots of people will agree and a lot more won’t about that boundary for “leftism”.

• The conflation of terms like liberal, leftist, communist… into one and the same is a topic deserving of its own dissertation that can only be explained as the resulting image from the warped looking glass that is the current American political landscape, concept that is often illustrated by talking about the shift of the Overton Window. These things in turn can be explained as the lasting echoes of McCarthyism and its Red Scare tactics that had a profound effect on American political discourse.

• Liberalism (another term so broad it would be impossible to fully explain in a few sentences) in its modern conception, and especially as “liberalism” is understood outside of the US, would mean an adherence to market economy ideology and the belief in private property. That would include all the range of positions from “The system is fine just as it is” to “The system is inherently fine it just needs some minor touch-ups” and all of them would find themselves opposed to leftism, which following the analogy would be the position saying “The system IS the problem”.