By and large, the support for liberalising drug policy comes -- unsurprisingly -- from the liberal left. While support for the War On some Drugs comes from the right. However, like almost all left/right analyses, that's extremely simplistic and only tells part of the story.
In the US, for instance, there's a sizeable chunk of the right wing who identify with the libertarian ideal of small-government / less central interference with local legislatures.
So, by granting individual states more power over cannabis policy, Obama is -- rather cleverly -- advancing a left-liberal agenda while simultaneously splitting the right, by appearing to appeal to libertarian principles while he's doing it.
It's a good move and worthy of support. But it does beg certain questions (like: If Obama thinks it's right for Oregon to set its own laws regarding pot, why doesn't he think it's right for Mississippi to set its own laws regarding abortion?)