National parties mediate the differences between their regional bases. The Democrats, for example, must negotiate between the interests of their constituencies in the northeast, the upper midwest, and the west coast. What an autoworker in Detroit sees as a critical political issue from a taxi driver in New York City, and both would likely disagree with a barista in San Francisco, or a farmer in North Dakota. The result is often a mishmash of both policies and politicians: Byron Dorgan, Democratic Senator of North Dakota, holds substantially different views than Barbara Boxer, Democratic Senator of California. This is a good thing in electoral terms, as it enables political parties to contest and win national elections.
Regional parties, by contrast, are usually much more consistent (though not entirely) in their ideology, policies, and politicians. This gives them a stranglehold on their particular region. But it becomes a reinforcing cycle: the politicians that emerge from a regional party are the ones that are successful in that region. Politicians who do not adhere to the ideological template are marginalized or forcibly evicted from the party. And with each success and each eviction, the party regionalizes itself further and begins the cycle all over again. This is a bad thing for political parties, as it makes it difficult for them to contest national elections and weakens them everywhere but their particular region.
This syndrome is currently at play in Pennsylvania, where Republican Senator Arlen Spector is in danger from his own party. Specter’s relative moderation no longer seems to fit within the increasingly conservative and increasingly Southern GOP, and so he will be challenged in the Republican primary by Pat Toomey, who’s social conservatism fits much more closely with the current template. If Toomey wins, and it seems likely that he will, it is extremely difficult to see him winning against any reasonable Democratic candidate in a state that went for Barack Obama in 2008 by over 10 percentage points. Even if Specter wins, it will be by moving to the right, something that will weaken him in the general election. The likely flip of the second Senate seat in Pennsylvania to the Democrats will continue the regionalization of the Republicans, which will in turn make it more unlikely that Specter-like figures can survive in the party. The big tent is a useful electoral tool; diversity helps win elections. If the Republicans’ tent continues to shrink, there will be less and less room for actual voters.