I suspect Sanders would have higher impact in a multi-party system, because apparently there is a group of maybe 30% of Americans who would be his fans and could vote for a progressive party under his leadership.
What I would really like to see is a comparison of the political systems, across different countries perhaps. To take an issue and compare how it was pushed through the different systems. I can't help it but today, EU seems more progressive (in the sense of trying to make life of citizens better) than the U.S. (and UK).
That’s where I disagree. I don’t think the percentage is anywhere near that high. Instead, I’d say that by latching onto the Democratic Party apparatus, his ideas have become far more popular and relevant in the party (and subsequently the country) than if he was running the Green/Libertarian party equivalent.
In the 2nd sentence, you're assuming exactly the conditions of the two party system. In Europe, new parties (like Greens or Pirates) were successfully able to bring new issues to the traditional political divide.
It seems clear to me that both Trump and Sanders (outsiders / fringe movements) have had an immense impact on their respective parties, far more than they would have with a separate political party.
To use another example: the two party system ensures that everyone is in the same room and has to use one of two microphones to speak. Since there are limitations, everyone has to compromise: the establishment and the newcomers.
A multiparty system lets everyone have a microphone (which drowns out all the voices) or lets you simply go to a different room. Both don’t require much compromise and I don’t see either as effective for maintaining the union.