David B. Rivkin and Elizabeth Price Foley have risen to the defense of the attorneys general of Nebraska and Oklahoma, who complain about spillover enforcement costs from Colorado’s law legalizing marijuana and hence are asking the Supreme Court to declare that law unconstitutional (“Federal Antidrug Law Goes Up in Smoke,” op-ed, Dec. 29). Along the way they respond to conservative charges of “fair-weather federalism” by pointing to President Obama’s failure to enforce federal drug law in Colorado as the real problem. Yet the gravamen of their legal argument is that, under the Constitution’s Supremacy Clause, federal law...