On American political parties held captive by their interest groups and ideologies

In the comments to Dark Age America: The End of the Old Order, Purple Tortoise asked the following.

Given the widespread obvious discontent, I have been surprised to see that no mainstream politician has really gone for grabbing the populist center and run on reining in the banks, restricting offshoring and immigration, and ceasing involvement in foreign wars. It seems like it would have been a great vote-getter to me. What am I missing?

Tortoise was quite obviously playing with Greer's idea that fascism is a totalitarianism that grows out of a disenfranchised center, a topic I elaborated on in The Archdruid on Fascism, part 2.  Neither Greer nor anyone else answered him directly for five days, so I decided to chime in.

What you're missing is that none of the top four parties in this country will support that combination of positions. Each party will support some or even most of them, but will find at least one of them anathema, so there is no home for that platform among them.

The Republican Party won't consider reining in the banks or ceasing intervention in foreign wars; the closest one got to those positions in the GOP was Ron Paul, who was interested in "Ending the Fed," but I don't know what he thinks about other big banks. I don't recall his son Rand continuing that policy. Both are still anti-interventionist, and both make the mainstream power brokers in their party furious. Note that Ron Paul didn't even get as far as either Rick Santorum or Newt Gingrich in the 2012 primaries. [His son Rand] is going to have an uphill fight getting through them in 2016 as well.

The Democratic Party won't support strong restrictions on immigration; they're having enough trouble on illegal immigration. Restrictions on immigration would be considered an expression of racism, and the Democrats include racial and ethnic minorities in their coalition, some of whom would be most displeased by an anti-immigrant stance.

The Libertarians would find restrictions on the banks, outsourcing, and immigration against their principles. The party actually advocates for open borders for both money and people.

The Greens might go for restricting immigration except that they don't want to be seen as racist, either. Since being pro-redistribution and anti-racist are both Left positions, it's hard to be anti-immigrant and be seen as Left in the U.S. these days.

The largest party in the U.S. who might be for all those things is the Constitution Party. Any politician in that group who can wrap those ideas in a flag while carrying a cross will get away with them. Pity the party is the personification of amateur hour in politics.

Neither Purple Tortoise nor Greer had anything to say about my comment, but they had the excuse that it was approved on Wednesday morning, and Greer and all of his readers were probably already looking forward to his next essay.

Just the same, consider Purple Tortoise's question and my response an explanation of why, despite Duverger's Law working, people are frustrated by the major parties and may not even be able to find a fit in the minor parties.  Not all possible combinations of positions can be accomodated within them, so those who do have those collections of views have no one to represent them.  Getting rid of the current two major parties won't solve that problem.