Blankenhorn (2018) identified no less than 14 causes of political polarization in the United States:

To note, the continuing and widespread upshot of economic decline from the 1980s, notably the destruction of the country's manufacturing base, does not feature in Blankenhorn's (2018) top 14. (Nor, for that matter, do any economic drivers of polarization.)

Reference

Blankenhorn, D. (2018, May 16). The top 14 causes of political polarization. The American Interest. Available: https://www.the-american-interest.com/2018/05/16/the-top-14-causes-of-political-polarization/

Thank you for that link, it's very interesting. I think the listed causes are not all on the same level. I argue that the main reasons are:

* high and rising inequality

* individualised, globalised, complex, accelerated societies leading to uncertainty and to a strong need for belonging and conformity

* different unsolved or new crises

* new media

No other than The Economist magazine has charged social media with invading privacy, spreading propaganda, and undermining democracy ("Pessimism v progress", 2019). In a volatile, uncertain, complex, and ambiguous (VUCA) world characterized by growing economic inequality, not to mention climate change and environmental degradation, social media platforms that pose a fundamental challenge to the traditional authority of the state and its institutions must surely be included among the primary drivers of polarization.