If I were a Mexican drug cartel, I would use my existing connections/influence in Mexico to use Mexican media to attempt to paint the picture: "Mexico thinks legalization will lead to more violence".
Right now a powerful argument in favour of legalization in the states is that it will reduce drug related violence, since the cartels will be forced out. Try to make it seem that Mexico disagrees. You don't even need to counter the "legal => less violence" argument with logic or facts; just use FUD combined with the inherent additional credibility you will have in Mexican affairs by being from Mexico.
Right now a powerful argument in favour of legalization in the states is that it will reduce drug related violence, since the cartels will be forced out. Try to make it seem that Mexico disagrees. You don't even need to counter the "legal => less violence" argument with logic or facts; just use FUD combined with the inherent additional credibility you will have in Mexican affairs by being from Mexico.