For at least the past couple of decades the mainstream media has had an obvious liberal bias, but the conservative politicians have understood that and largely played by those rules. When Trump arrived on the scene they went into overdrive, Trump called them out on it, and they declared war. I think that has fueled a lot of the division we see today.
Trump has brought this on himself, but some in the media may as well put on a black hoodie and call themselves antifa.