Alternative positions in the two-party system

I've always been intrigued by the American two-party system and its difference with respect to Continental politics. I think the comparison is very instructive and can shed light on recent political developments. Case in point: in the current issue of The Atlantic, Jonathan Rauch deplores the state of Americal politics, in particular the lack of discipline in both the Democratic and the Republican parties, whose "politicians, activists, and voters" are less and less likely to line up behind the party leaders. Terms and phrases in quotes are Rauch's.

Rauch explains this by the reform of the political system over the last few decades, which rendered the "middlemen" less powerful, and thus unable to defend against such "symptoms" as "insurgence" and "radicalization". The author really likes the allegory of the immune system fighing the "pathogens".

These "symptoms" are also common in Europe, where they gave rise to powerful political movements and parties. This has not happened in the United States, probably for the simple reason that, in a two-party system, it can be easier for new political orientations to work from within the existing parties, rather than create new ones, as in Europe. The religious conservatives and the libertarians have to go through the Republican party and the socialists through the Democratic one. As long as they were in the minority, this was not a problem for the establishment, but when the "outsiders and insurgents" start getting the upper hand, the "body politic" (which for Rauch seems to mean the political status quo) may not recover.

The comparison with Europe also casts doubt on Rauch's diagnostic: the radicals gained prominence at about the same time in the US and all over Europe, although the political systems are quite diverse. A much simpler explanation is that the sluggish economy and the resulting social problems caused the discontent and the anti-establishment feeling on both sides of the Atlantic.

To summarize: as in Europe, in the United States alternative positions became politically relevant in the last 10-15 years, probably for reasons unrelated to political reform; unlike (Continental) Europe, however, they had to go through one of the two major parties due to their stability, itself due to the first-past-the-post electoral system.