<<I think the largest explanation is negative partisanship.>>

It is certainly an important factor, not only in the U.S. but in other Western-style democracies, as well. In the U.S., evidence indicates that the racial realignment of the major parties is closely connected to it:

"For decades before Donald Trump came on the political scene, Republican-elected officials and candidates sought to lure racially conservative White Democrats in the South and elsewhere into the GOP camp with racially tinged messages about the dangers posed to Whites by African-American crime, forced busing of schoolchildren, and affirmative action and by emphasizing the complicity of Democratic politicians in these threats. ...[T]he American party system underwent a realignment that transformed the racial, regional, and ideological bases of the two major parties.... That realignment resulted not only in a growing gap between the racial composition of the Democratic and Republican electoral coalitions but a dramatic increase in racial resentment among White Republican voters" (stevenwwebster.com - Negative partisanship rabid)

More broadly, in-group/out-group identification and hostility have deep roots in human nature, and power-seekers often exploit those tendencies in pursuit of personal gain. Trump's nonstop ravings about masses of criminals, Hannibal Lecter, "poisoning the blood," etc., is a stark example.

The system-level problem is that negative partisanship infects the entire political process, making compromise and sensible policymaking increasingly difficult. That reinforces prevailing negativity, and the wheel turns.