“If he resigns for no plausible reason, the Dems will still lose the black vote.”
Maybe so. But probably not nearly as badly. Black anger would be able to focus on a particular person (the challenger in the primaries) in the case of a primary challenge. It would not be so focused in the case of a resignation. Zero would blame the Republicans (and, of course, George Bush) when he resigned so that would direct some of the anger to R’s.