They actually want Holder to tell voters that Prop 19 will be "void" even if it passes, as though no one in California has ever heard of a marijuana dispensary. This, in their view, is an argument so strong as to warrant an entire Wall Street Journal op-ed dedicated to it.
For another layer of absurdity, consider that one of the authors, Karen Tandy, presided over the DEA during a dramatic expansion of the medical marijuana industry that she now claims is legally impossible, even though it actually happened in real life while she was in charge of federal drug enforcement. She knows as well as anyone that California can make its own drug laws whether the DEA likes it or not. It's true that she could have prosecuted everyone in sight under federal law, but for a variety of practical and political reasons, that isn't what happened. It's not likely to happen if Prop 19 passes either, at least not unless Obama has a masochistic desire to further alienate his base as we heads towards 2012.