Decades of gerrymandered districts, voter suppression, power concentrated in the hands of only 2 parties, tribal us/them indoctrination where people vote party over person or policy, plus massive, unlimited, anonymous inflows of money into politics has allowed the craziest and most extreme candidates to “represent” us.
The Republican Party saw a wave of extreme representatives come to power after the Great Recession with the anger of the Tea Party movement, has spent years denying basic facts on topics like evolution and climate change and tax cuts paying for themselves and all of that fertilized the ground for conspiracy thinking and treating their opinions as if they are equivalent to other people’s facts.
This led then to an opening where the entire party experienced a hostile takeover in the 2016 election where Republicans were too afraid follow their conscience, to speak out against Trump, or to vote in favor of his impeachment. They put personal power over health of the nation and correctly calculated that their own voters liked Trump more than they liked them so they needed to fall in line or leave.
Now it’s mostly just a cult who thinks people on the left are subhuman garbage in need of a bullet to the face and these divisions are being actively stoked by state actors like Russia, China, and Iran and are being further amplified by US citizens themselves who have plugged into QAnon and other actual fake news sources.