Question: In the US, there are many large cities that are dominated by a single party. After Reconstruction and through the 70s, the Southern States were solidly Democratic.

There are often strong factions in these single party districts, but they do not become full parties with clear signalling mechanisms and separate governance structure. 

Why not? What are the theories of why local parties do not emerge?

Are there any examples of local parties emerging in the US?

Answer: 
blip's answer kind of hinted at an answer, but not to the underlying cause.

One of the major reason for polarization is that, as researchers discovered, " people want to live among people who share their ideology as well. People are motivated to move away from communities where they don't fit in and try to find areas that are more congenial".

This is discussed in detail in this NPR article: <http://www.npr.org/sections/itsallpolitics/2013/11/26/247362143/how-republicans-and-democrats-ended-up-living-apart>; but the underlying major study is summarized as:

> 
> But the U.Va. and USC researchers, in a forthcoming paper in the Journal of Experiment Social Psychology, suggest that increasing numbers of people want to live among people who share their ideology as well. People are motivated to move away from communities where they don't fit in and try to find areas that are more congenial.
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> Individuals have always sought to live among others they find congenial and similar to themselves, but, increasingly, that includes partisan leanings, Bishop says.
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> "It is natural for people to desire communities where they share a worldview with their neighbors," writes the team, led by Matt Motyl, a doctoral candidate in psychology at U.Va.
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Broadly speaking: demographics. In the US, rural tends to lean republican and urban leans democrat. We also have bigger regional leanings (like New England vs. Texas). On top of that, we have gerrymandering where a party that is in power draws the district lines in hopes they can stay in power. 

> 
> What are the theories of why local parties do not emerge?
> 
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> 

They do emerge, but are often at a disadvantage against candidates from the major parties, who find plenty of support being a part of the bigger machine. 

> 
> Are there any examples of local parties emerging in the US?
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> 

Yes. Some that come to mind would be:

* [The Independence Party of MN](https://en.wikipedia.org/wiki/Independence_Party_of_Minnesota) who famously got [Jesse Ventura](https://en.wikipedia.org/wiki/Jesse_Ventura) elected as governor.
* [Socialist Alternative](http://www.socialistalternative.org/) in Seattle who got [Kshama Sawant](https://en.wikipedia.org/wiki/Kshama_Sawant#Seattle_City_Council) elected to city council.
* Wikipedia also has a [long list of regional parties in the US](https://en.wikipedia.org/wiki/List_of_political_parties_in_the_United_States#Regional_parties).