As I understand it, one of the reasons Prohibition ended was that the Depression made the costliness of it more obvious.
(I have a mental category of "rich enough to be really stupid". It's not that being rich necessarily makes you stupid, it's that if you are stupid, you can afford to do it on a larger scale.)
The war on drugs has been a hideously expensive disaster, and if this fact can be made to register on the general public, this is the time to end the war. It may be necessary to find a way to phrase ending it so that people don't have to admit it was a mistake.
If Obama is looking for ways to cut the deficit in half, ending the war on drugs is a good place to start. The fact that he doesn't have a punitive temperament may help.
Here's one for my curiosity: is there a good overview of the effects of the war on drugs outside the US? I wouldn't necessarily use it as part of the campaign to convince the current administration-- I think Obama's empathy pretty much stops at the US borders, though he's enough of a pragmatist to listen if foreigners say their toes are being stepped on and they have enough clout to make it clear that they'll step on our toes if we don't get off of theirs. (I'm thinking of the recent kerfluffle about "buy American".)