There’s nothing inherently wrong with political partisanship.
Being a partisan focuses the mind, giving it form, orientation, and direction when engaging in political deliberation and action. Partisanship expresses a conviction about the whole of things — about justice and its demands, about the common good and its requirements. If everyone in the political community agreed about the demands of justice and the requirements of the common good, there would be no partisanship and therefore no politics. Public officials would just be competent managers overseeing a staid, automatic, and uncontroversial process of allocating public goods. But of course we do disagree about justice and the common good. And so we have politics, and so we have partisanship.
But partisanship without limits — well, that is a problem. And today, both sides suffer from it.
The limits of partisanship have disappeared — with many on each side of the partisan divide burrowing so deeply into their distinctive view of justice and the common good that the concept of a truth independent of partisanship has begun to warp, bend, and even break altogether. Or rather, each side has come to equate its own partisan view with the truth as such.