What are the roots of the current divisiveness of American politics/discourse?

[Orser67]

Major factors:
1)America is not any more divided now than it has been in the recent past, it just feels that way: I actually think that the relative bipartisanship of the 1940s-1970s is the historical exception to the rule, rather than the standard
2)Lingering effects of Great Recession/Economic Inequality: I'd point more to the relative decline of the U.S. economy since the 1970s, as other countries modernize and take advantage of low trade barriers
3)White men threatened by the rise of women and minorities: I'd break this down into three factors: Whites threatened/angered by government programs aiding minorities (especially blacks) since the 1960s, immigration, and the feminist and LGBT movement alienating cultural conservatives
4)Lack of a common existential threat - USSR, Nazis, Al-Qaeda: The lack of a common existential threat since the end of the Cold War, specifically, although al-Qaeda filled that role in a lot of peoples' minds for about a year

Another factor that comes to mind is a broad disaffection with government since the late 1960s, which I think is to some degree a phenomenon independent of the other factors. This led to a separation between those who see the government as a force for good and those who see it as basically a parasite (and of course there are other groups with more nuanced views)

Moderate factors:
5)America is an empire in a post-imperial world - our large and diverse country naturally lends itself to increased division: I don't know about the post-imperial world part, but the diversity is a challenge (as well as an asset)
6)The media is incentivized to promote conflict and sensationalism
7)Social media has strengthened the "bubbles" we live in, by showing us hundreds of people who agree with us and little else
8)A general moral decay, due to increasing irreligiosity and secularism: I don't agree with this statement. But I agree that the decline of Christianity has made many feel threatened, which goes into point number three above

Negligible factors:
People feel less agency over the decisions made by government due to increased influence of pan-national organizations like the UN and WTO: I would think that the rise of the bureaucratic state has affected "agency" far more than the rise of intergovernmental organizations. I think only the ideological fringe really cares about these groups.

Foreign powers have nurtured divisions between Americans to weaken the nation on the international stage: The only country that has done this successfully to some degree is Israel (specifically Netanyahu), but it hasn't had that big of an effect. Russia meddled in the election but I don't think that that has appreciably added to divisiveness (at least not yet)