The only options available for left-wing party affiliation in the US are Democrats, Greens, Peace and Freedom Party, and the Justice party. the Justice party and Peace and Freedom Party currently have very few or no actual representatives or presence in current politics. Adding more prominent/current Socialist parties (like the DSA and Socialist Alternative), or even a generic Socialist party code would more accurately reflect the current US political landscape.
Thanks Adam! Let us know any specific parties that you'd like to see below.