Marijuana is prohibited by federal law, and by the laws of most states, including Colorado, until recently. When the voters of Colorado passed the ballot initiative to legalize pot, they were revising state law. Federal law remains unchanged.
If Pelosi is suggesting that the federal government should not enforce, or require the state of Colorado to assist in enforcing a federal law in a particular state because the voters of that state voted to amend their states laws, doesn't this implicitly set state law over federal law? How is this any different from "nullification" of federal gun legislation?
Whether a person agrees with liberalization of drug laws or not, it's a REALLY bad idea to start allowing states to decide which federal laws can be enforced within their jurisdictions.
This is actually something I've thought a lot about because while I've slowly come around to the pro-legalization position, I also think federal supremacy is the bedrock of our governmental system. And you don't just toss it aside when the breach is something you support on policy terms.
On this particular issue, the issue here is not nullification. The federal government has its laws and the states, independently, have their laws making marijuana illegal. The states aren't under any obligation to have those laws. It's quite different from abortion, for instance, where the Supreme Court has prohibited states from having and enforcing certain laws. The state, as far as I know, is also not barring the federal government from enforcing its laws within its borders, which is what nullification actually is.
Still, legalizing use and ownership within states and creating mechanisms to allow things that are illegal under federal law does nudge up against the supremacy of federal law. As a practical matter, what I'm not totally sure of are the narrow legal obligations of state law enforcement to assist in the enforcement of federal law.
So, on this whole issue, put me down as uncertain and conflicted. I didn't use to be pro-legalization. I am now. But the supremacy issue is an entirely separate matter.