Where did the tribal divide start? I'm thinking it started sometime after 9/11 when Jr. told the world that you are either with us or you are against us. That started the "us vs. them" mentality.
It was further exacerbated when people in this country started to blame Muslims for everything. The "us vs. them" mentality was getting stronger.
Then, when Obama was elected, it was even worse. They didn't like him, he had a funny sounding name with a middle name of Hussein. Lots of people tried to say he was a secret Muslim and used that as a way to denounce and try to undermine him. It was also helped along when McConnell got all the GOP together the night Obama was elected, and swore to make him a one term president.
When people on both sides saw that people were becoming used to the rhetoric, decided to ramp it up even further, using scarier and scarier language until the terms had become hopelessly mixed up. Politicians started to use scarier and scarier images, even though most of them would never occur.
And, the people got used to how their politicians were talking, and now, you can't hardly talk to the other side, because they don't want to believe you due to tribalism.
And......................to tell the truth, I don't know how they will stop it.