Dana Milbank has come to believe that politicians in the US have become more partisan as the electorate has become more partisan. The electorate is less informed about issues than it is about what is bad about the "other team". Elections have become more tribal over time. Politicians in safe districts, or safe states, increase their rhetoric about the tribe that they dislike. That turns out the vote for their side. The key to victory is getting your tribe out to vote, or in making it more difficult for the other tribe to vote. It is not surprising in this scenario that politicians are less able to compromise.
The tribal behavior of the electorate is also reinforced by a more partisan media. The "mainstream media" which makes an effort to be bipartisan, becomes the enemy of the tribal media (and the social media) which reinforces the belief system of its tribe. The divisions between the rival tribes also grows along with the rising amounts of advertising expenditures on political campaigns. The electorate becomes more tribal with each election cycle and political campaigning becomes perpetual in Congress. The political parties also become more dependent upon those who fund their campaigns. Over time the political parties will become less relevant. Tribal identification is stronger than political party identification. Its hard to imagine how this process of tribalization can be overcome.