Doug, you paraphrase my quote from Derrida as follows
'opposition to racism, totalitarianism, etc., in the name of *spirit* or universal human rights or some other axiom (i.e., something taken as self-evidently true, or assumed from the outset),' is not convincing.
This is your version of Derrida's list: '*spirit* or universal human rights or some other axiom '
But this is Derrida's list: 'in the name of the spirit, and even of the freedom of (the) spirit, in the name of an axiomatic - for example, that of democracy or "human rights" '
You dropped democracy from Derrida's list. It seems to me that democracy is not a delusion, but a very real foundation from which to criticise fascism. It is the basis on which one ought to criticise fascism.
You say "What you quote certainly doesn't prove that fascism and democracy are as bad as each other in Derrida's eyes." But that is precisely the argument of Of Spirit. There he isolates what he takes to be malevolent in fascism, and it is 'Spirit' - the very thing that it has in common with democracy. Subsuming them both under the commen heading Spirit is his way of equating them.
You say: "these abstract universals don't exist. They're creatures of imagination, of social convention. I would have thought that a Marxist would find this point obvious."
Only a very poor Marxist. Marx himself rubbished the idea (as for example in his treatment of the proposition that money was a social convention - Galiani's, if I remember right). His point is that they are social *laws*, and as such are no less real than physical objects. Value, is, as he says, without an atom of matter in it, but nonetheless real for all that.
Marx does not think that universals are creatures of imagination. Labour is, as he says, an abstraction, and a real abstraction, an abstraction that takes place every day in the labour market.
Who is it that thinks that democracy is just an illusion, the kind of thing that only naive people would believe in, that ought to be swept aside? Sounds to me like the point of view of the Caesarist Schmittian, all very Martin Heidegger, and not Marxist, at all.
It is a common error to equate social constructionism with Marxism. Ten years ago I wrote an article distinguishing the two (published in a collection edited by Suke Wolton, Marxism, Mysticism and Modern Theory).