You also used the term 'parliamentary system', but the US lacks that in one sense of the word. No one is going to take a minority faction into Congress and build a mass political movement of ideas and policies outside the Republican or Democratic parties.
Yet, since so many ideas that affect the evolution of the relationship between government and society have to come from the left, this is a sore lack in the US. Do you think all the people who voted Green in Europe are dyed-in-the-wool Greens devoted to that movement to the end? Or those who voted for right wing politicians in Austria, for example? Charles Jannuzi