Lee Drutman wrote for Vox's Polyarchy about how elections today are both highly competitive at the national level and extremely uncompetitive at the state level, and why this signals an unusual shift in US political history.
Our political institutions are just not set up to handle two deeply polarized, highly competitive political parties. And the current partisan division — in which racial, cultural, and geographical identities all line up with partisan allegiances — is just deadly for democratic compromise, since it organizes politics around a set of national identity issues where compromise is impossible.
Moreover, because our national elections are so tightly contested, majority control is always in reach or in danger for both parties. This breeds a particularly nasty zero-sum type of politics, in which compromise becomes difficult to impossible: Why help the other party when your electoral success depends on their failure?....
....A two-party system can work when both parties are moderate, centrist parties that appeal to the entire country. It can’t work when the parties are regional parties who appeal to very different parts of the country, which effectively divides politics along cultural and identity lines. That’s what’s happening now.
Maybe something will give, and American politics will get the major realignment our party system needs. But our political institutions right now make it very difficult. They are simply not set up to handle our current political divisions. And they are breaking under the strain.