Self-determination theory says autonomy, competence, and relatedness are our three most basic needs. I feel that the US party system sees to these needs. Our right to vote and select delegates gives us a sense of autonomy. We feel related amongst peers who may share some of our opinions and who are of the same party.
The two major U.S. political parties have stances that are not a result of our basic psychological needs. Although both parties have been accused of excessive stances, it is actually the Republican Party that has taken extreme right wing positions. Our basic needs are safety, food, and love. The party appeals to greed and depriving the poor.
The excessive stances of the US political parties cannot be justified by arguing that they result from basic psychological needs. If that argument stood up, any criminal action could be excused by saying it springs from a deep-rooted psychological impulse. The US political system is broken and needs a fix in a hurry.
For the most part, the political parties in the US are relatively neutral. If slightly "excessive" stances present themselves in candidates, those candidates eventually lose out to the more neutral parties -- i.e. currently the Democrats or Republicans. While the outliers in stance make for better news articles, it's still the average that gets elected.