One thing that is interesting is that Congress usually blocks funding for DC laws by restricting how we can spend our local tax dollars (through the appropriations process).

When DC decriminalized possession, Rep. Andy Harris of Maryland tried to block the decriminalization by blocking funding for decriminalization. If that sounds like the legislative equivalent of a double-negative, it is!

Harris's legislation potentially would have had the un-intended affect of preventing the District from spending any money on enforcing the decriminalization: no money could be spent on writing citations, printing tickets for possession, or prosecuting cases in court. This would have effectively legalized it since the criminal law is already off the books.

http://www.washingtonpost.com/blogs/mike-debonis/wp/2014/06/...

I doubt Congress will pick this up. They have 90 days to veto DC laws or they go into effect, and they can barely name post offices at this point. Obama has also promised to veto the veto.