To be fair, the Democratic party is not a party in the sense of the 'normal' European party democracies. The American duopoly is quite a bit different to most other "liberal democracies". Standing as a Democrat requires running in the Primary system, not a party membership or attendance of conferences or whatever. Dem/Rep are more like wrappers over a formal two-party system like the Communist Party is the wrapper of a socialist one-party democracy.

Minor parties fulfil a similar role in the US as they do in something like China, they have low to no access to the levers of power and never will, and are only allowed in so far as they serve no real threat to the duopoly/single party democracy.