An email from a reader:

The question whether Rubio would enforce federal marijuana laws is artificial. The federal government cannot enforce its marijuana laws without a massive expansion in the federal police force. It depends utterly on the states to enforce the federal pot laws on a voluntary basis, and unlike other instances of “cooperative federalism” this is one example where the feds really have no coercive lever. State willingness to enforce federal drug laws is purely voluntary, because the feds have enacted a prohibition that Congress has provided no means of the feds’ enforcing directly.
Would Rubio adds hundreds thousands of beat cops to the FBI and ATF? Because if not, then he would not enforce the the federal marijuana laws. The real question is whether he thinks the states have a duty to enforce federal drug laws. And the correct answer is that they do not. The progressive-anti-federalist answer is that they do.

The email reminded me of Mario Loyola’s look at marijuana federalism in The Atlantic from earlier this year, which is worth a read.