Post Newt, we had 9/11, after which we were unrecognizably united (by current standards) for a period of months, even years after that. One of the more entertaining aspects of the '16 race was watching Trumpies basically mock that unity by going after Hill for voting to authorize the invasion of Iraq. That was a hoot - a bunch of neocons turned populist nationalist mocking a vote during that period of unity (haven't seen it again), when the benefit of the doubt went to the Commander in Chief.

The divisiveness today is less the result of any humans than of media technology - Internet and social media - which is advancing far faster than our ability to adapt.