The root cause is economic. Wealth is being transferred to the billionaires at a growing rate. This causes difficulty for most people. If we were smart we would unify and tax obscene wealth, and everybody would be happy. But instead, those billionaires spend hundreds of millions of dollars stoking the flames of a culture war. They amplify every emotional issue to motivate emotional (outrage/anger/fear) voting. Now instead of liberals pushing progressive tax policy, they just vote for moderate democrats because the only other choice is a fascist. And if it wasn’t that, they’d stoke liberals into a Green Party socialism craziness and vilify it, so that again the sensible choice is moderates.

The real question you should ask is why are most representatives on both sides millionaires? There’s really only a handful of congress members who don’t already have millions in assets. I’ll answer it for you - it’s almost impossible to get elected without a major campaign donation. Rich people fear poor people rising up, so they throw money at measly millionaires to do their bidding and keep their power.

In summary: the rich control 90% of who gets elected on both sides. The rich get whatever they want, because everybody else thinks they need to vote based on a couple issues they saw on TV. The actual candidate doesn’t matter. They’re all billionare’s puppets, who vote on party lines in order to get re-elected. Stir the pot and lose your position.