I have real concerns about what the federal government might do if California legalizes marijuana (and I do think there is a good chance but not by 61% as the OP dreams about). I think Obama could use us to have his "sistah souljah" moment. Bastard.
I thought about the same issue of what the feds would do if California formally as opposed to de facto legalized pot, as is the case now. My surmise is that if state legal authorities don't enforce federal law regarding pot in California, I don't think it is practicable for federal authorities to do it on their own, which leaves cutting off federal funds to California to bring it into line, and I just don't see that happening. So as a practical matter it appears to me that California would hold the trump cards under this scenario.
Also the feds threatening to cut CA off from federal funding won't matter if the proposition passes.
Do you really think that the money that would come from taxing pot would off set the loss of federal funds?