Most liberals have what I would characterize as a deontological opposition to discrimination. That is, they think that discriminating against or maligning someone on the basis of membership in a protected class — women, trans people, black people, and other racially oppressed communities, etc. — violates a rule that should be inviolable.
In this view, such discrimination (be it legal, or expressed through hate speech, etc.) is not just wrong because it has bad effects, or because it harms members of the groups in question; it’s wrong because we have a duty to treat humans as equals, and it is never acceptable to violate that duty, even when doing so seems politically expedient.
This mode of moral argumentation came through in the Rogan controversy when Sanders and Rogan’s critics took pains to stress that accepting a Rogan endorsement was not merely unwise but immoral, and that these two judgments were distinct. Accepting the endorsement was not wrong because it hurt more people (by amplifying bigoted speech against vulnerable people) than it helped (by increasing the odds that a pro-trans Sanders administration comes to power); it was wrong because it is wrong to coddle and amplify bigots, full stop.