In truth, believe-victims activists have been making generous use of the motte-and-bailey fallacy. This is a form of argument in which a person makes a strong, unreasonable, and indefensible claim—the bailey—and then falls back on an uncontroversial claim—the motte—when challenged. With "believe victims," the bailey position was something like what Biden and Clinton said: Presume that each and every alleged victim is telling the truth. The motte position is closer to this: Respect and support alleged victims, and don't automatically discount what they say. In the wake of Reade's allegations against him, Biden has unsurprisingly retreated to the motte.This is a topic that Slate Star Codex has treated repeatedly over the years.
I'm not sure it's properly speaking a fallacy so much as an objectively dishonest rhetorical strategy. A fallacy is an error in logic; informal fallacies occur in ordinary rhetoric, which isn't usually amenable to the strict logic in which formal fallacies occur. You can get a formal fallacy in rhetoric, it just doesn't happen much: but if I make a claim that P -> Q, !P, therefore !Q, I've made a formally fallacious argument. If I argue that you're a bad person therefore you can't be right, that's informally fallacious. In both cases, I'm making a claim that doesn't follow from the premises.
The motte-and-bailey is only an error if you don't notice that you've shifted your goalposts substantially. Otherwise, it's a lie. Reason also uses the term 'gaslighting,' which I learned from Tex, and which is a form of intentional deception.
So the question is whether or not they notice their own shift. Maybe not; progressivism is based on fervently asserting beliefs in things that you probably have to know are not true, e.g., that all people are per se "equal" (rather than possessing one form of political equality). Maybe at some point you just don't notice that you've shifted from really saying 'believe all women!' to 'don't just dismiss women'; or from 'it is sexist not to build systems biased in favor of women' to 'feminism is just about equality!' (And which equality, again?)
Eric Hines accuses me of being too generous to my opponents. Perhaps I am; but I do see a lot of self-deception in humanity. I think many of these people really are in error rather than intentionally lying; I think they really can't see outside the lies on which they've founded their lives and their vision of justice. It's a big problem. It's hard to reason with someone who is lying to themselves all the time about the very questions you're treating, especially when (as here) they have gigantic social support systems to reinforce the lies and to protect them from having to grapple with the fact that they are engaged in a (self?) deceptive practice.
