First of all, there is no “Left” in America. Or I should say none which has any political power. The power of the American (USA) left was in organized labor. Now, except for skilled trades and even professional classes, organized labor has no power at all. This is because it has lost its bargaining strength. And off shoring of jobs and automation have everything to do with that. Even immigrants contribute to the problem, but in no way near to the degree of the first two items mentioned.
Second, because plurality vote victories are allowed in America (just as they were in Germany at the time Hitler came to power), there can be only two real political parties. Voting for any other than them all but guarantees the opposite of the effect desired.
And, Third, because there are only two real political parties, both can play to a base different from the one they actually serve. The Republicans play to a base of religious zealots and bigots of various stripes. The Democrats play to identity groups such as gays, Latinos, blacks, and other minority groups (which not surprising, the Republicans play against). Both parties actually serve the interests of the super to the near wealthy. This is not because they are evil. It simply makes good business sense. Elections can’t be won on wishes. It takes money to get the word out.