But remember (and be thus reassured) that the Europeans have parliamentary systems, where third parties often join coalition governments with one or another of the major parties. Third parties cannot achieve anything like the same level of influence under the American system.
If the Green Party continues to grow it will only serve as a more effective spoiler for 'Rats. If it grows large enough it will only pull the 'Rats yet further left as they attempt to woo it's members back into the major party fold. Granted it won't be a good thing if more Americans hold to these extremist/feckless views, but the consequence will be politically painful for the left, which is a happy mitigation.