Most Americans bemoan the confrontational politics of this country's two-party system. It was not always this dramatic. The image above summarizes data presented in a two-minute gif that I can't embed, but which is viewable here. I'll embed three screencaps:
The dots represent individual Democratic and Republican congressmen; the lines between them show how often each pair votes the same way.
There is a reasonably informed discussion at the gif site. I heard an excellent podcast on this subject, from either This American Life or Radiolab. They traced the acceleration of the us-vs-them mentality to Newt Gingrich's 1994 Contract with America and the rise of Rush Limbaugh on radio. Cable television has played an obvious role.
Things were not always wonderful in the old days, however, when Senators and congressmen passed bad legislation and collaborated to strike deals that ignored the wills of their constituents. There is no easy answer.