Assume for a moment that California voters approve Proposition 19 on Nov. 2. The state will have just enacted a process for legalizing, regulating, and taxing marijuana use that no one else in the world has ever attempted. But Attorney General Eric Holder, President Obama’s top law-enforcement officer, has said the administration will “vigorously enforce” federal drug laws in the country’s most populous state regardless of the vote. For all the trails that approving Prop 19 would blaze, much of its impact would depend on the extent to which Holder follows through on that threat.