"the left" and "the right", or "the democrats" and "the republicans" in essence, is too wide a net. "The left go off the deep end" is also, when you look into it, a fairly small percentage of the wider population.
But it feels like people are pushed to the fringes, because the Other Side is being pushed to the fringes. There's just, from the perspective of an outsider, a lot of antagonism between the two "sides".
And that antagonism is being fueled by someone. There's people with a lot of money and / or special interests who benefit off of the infighting and polarisation.