Social media is now a primary target for foreign election interference, I'm sure that's a major cause. In 2016 Russia targeted Clinton and supported Trump. But they also generally tried to increase division. One ugly thing about that is disinformation campaigns would actually get support from some Americans.
The House is Democratically controlled, but Republicans control the senate, so there's bipartisan agreement on this. https://www.nytimes.com/2018/02/17/technology/indictment-rus...
https://www.intelligence.senate.gov/sites/default/files/docu...