And yet there is almost no money being contributed to the oppositions coffers, take CA prop 64, for example:
Total campaign cash
as of October 2, 2016:
Support: $16,970,725.67
Opposition: $2,026,501.16
https://ballotpedia.org/California_Proposition_64,_Marijuana_Legalization_(2016)
None of the opposition funds appear to be from Pharma either.
Because pharma doesn't need to, or want to expose interests so blatantly or expensively. In such an environment, big pharma gets to hide behind candidates who align with their goals of slowing legalization, rather than directly opposing measures in states where they have no hope of winning.
Think about it from the perspective of pharma: you can either spend a bunch of money, multiplied by the number of states with ballot measures, to attempt to defeat a campaign which has wide support by those voting, or you can spend a little bit of money on a few campaigns in states where there aren't ballot measures available, or on representatives who will be in office for years, and the best part is that you only have to invest occasionally, since incumbents are hard to unseat. On top of it, you never have to publicly disclose your interest, unlike in campaigns against ballot initiatives.
Let's say that pharma were to invest 20 million into California. That 20 million represents a hundred 200k campaign contributions, or many more lesser contributions. Such contributions buy many more officials, ones with clout and respect, to protect their interests on a much broader field rather than a single state.
On top of all of that, you ALSO get politicians who are going to listen to other agendas of your company, deregulation, regulatory capture, creating or improving barriers to entry for competition, the list goes on.