Pick a handful of communities in the legal states: Wealthy, suburban districts that went blue, but narrowly. Send a couple of guys to each one with ten pounds of marijuana and ten thousand dollars in cash. Have them recruit some bored 21-year-old stoners, give them each twenty bucks for snacks and a dozen rolled joints, and have them hang out at the skate park or movie theater or wherever and just pass out free weed.

Those joints will inevitably wind up in the hands of teenagers and even young children. Ramp up the operation enough, and you've got a full-blown weed epidemic. Without any further prompting, parent groups will petition the state and federal government to intervene, and if not repeal the law then at least impose enough additional restrictions to keep an illegal operation cost-competitive with a legit one.

Of course, there are lots of ways this strategy could fail or even backfire.