The majority of votes come from the Center of America. See the Bell curve
in my article,
or in http://www.holisticpolitics.org. I disagree with this chart a bit in that I don't believe the center of America is represented by the Status Quo. More on that later though.
What happens if a left-leaning party, like the Green Party, runs in an election? The Greens take votes from the far left. Maybe you agree with them politically, but the majority of Americans don't, and that's why they can't win big elections. Even worse, by running, they allow the Republicans to win by forcing Democrats to tack Left to regain the lost Green vote, losing the Center Electorate.