People generally hold sweeping, often negative, opinions of "those people". Over the decades, "those people" has been defined by race, religion, ethnicity, etc. Today in the US it is which political party people perceive you to be affiliated with.

I call this a feedback cycle because in my experience the people using this sort of rhetoric aren't interested in being proven wrong. They have chosen consciously or otherwise to vilify a group, or buy into preexisting vilification. Undoing that is hard work so it just doesn't happen. Absent any pressure, people assume themselves correct and chalk up another datapoint confirming their own biases.

This is readily apparent in response to any mass killing/tragedy. My more politically extreme friends seem to race for any evidence they can find that they perpetrator is Democrat/Republican. Of course this boils down to pure speculation (gun ownership, smokes pot, etc), but the urgency and desire is there all the same.

In summary it is little surprise that the politicians in Washington are so hardlined on their stances and party affiliations. This is effectively the government the voters have demanded. When the citizens of a country start vilify half the country, their representatives reaching across the isle is political suicide. And thus we sit.