Has it always been this way, this lack of reason? It feels more extreme, or maybe I’m just more aware.
You can date it from various points in politics, and the right doesn’t have clean hands either. But throughout my lifetime, the left has always been nastier to the right than the reverse, in this regard.
The recent digression in civility appeared when the left decided they could win elections by taking a broad brush and painting the entire GOP as “racist” and “mysogenist”. The politicians and media accessories who did this knew and still today know that it was a tactic not based in truth; but it was eaten up by the left’s fellow travelers, who accepted it as gospel truth.
And, if you are working from the assumption that your debate opponent, by nature of their partisan affiliation, must hold and/or approve of morally reprehensible views, you have just written yourself a license to be as nasty as you like.
Now, that tactic is in the process of backfiring.