There are still Communists who claim that Communism is the perfect system, it just hasn't been tried anywhere (I'm not going to link for them; just google "CPUSA" and/or "wacko").
Soon (or maybe yesterday) there will be Conservatives who claim that Conservativism is the perfect system, it just hasn't been tried anywhere (I'm not going to link for them either; just google "GOP" and/or "wacko").
They both suffer from the same religious faith in an untestable proposition, they just differ in the details. Communium/Conservativism failed in the USSR/USA because of the human failings of the Kommissars/Republicans.
In fact, at the heart of both religions are the same thoughts: Humanity is defined by economic motivations, and we just need a Leading Party to implement the Ideal Economic Plan for success.
It doesn't work.
Partly the problem is that we humans aren't solely economic critters; we are more swayed by feelings of community, love and family, for example. More importantly: Any system that depends upon a political party to put it into practice it without skimming something off the top for themselves cannot be implemented by human beings.
The lesson for Liberals/Progressives/Etc is not to rely upon the Democratic Party ... help it, sure why not? it's a very useful tool ... but on community and democracy (small "d"). Progress will be slower that way, but less prone to corruption.