I'm sure the diarrhea merchants are having a field day with that "nation of cowards" line. I understand the desire amongst white folks to not be typecast as the scheming, sniveling villain. It's simplistic, and dehumanizing. It reduces individuals into an indistinguishable, robotic mass.
That said, you could only be offended by that line if you think "the nation" only includes white people. For the record, given the behavior of a lot of "black leaders"--and black people--pre-Iowa, cowardly ain't exactly wrong. Anyway, I really have no idea what Holder meant, beyond what he literally said. I found his speech unremarkable and vague. But I can only take that line as it was--a statement about the entire nation.