This is largely from direct observation back then, e.g. in 3rd grade a bit before "War on Drugs" was coined I had to write a thorough workbook detailing all the major drugs being abused then, why they were bad for you, etc.
And, errr, here you posit "total - non-trivial number < most" (if I'm not too tired to express that); on its face it's just an impression you have. Very possibly a correct one, but then again you admit there are anti-War on Drugs Republicans. And in theory one of them could do a Nixon in China ... but we all know how a lot of Democrats would reply, no matter what their personal preferences were.
I would posit that you have a significantly higher percentage of pro-legalization Democrats than Republicans. Most opposition to legalization comes from Republicans.
Consider H.R. 499, the bill to legalize marijuana. It had 16 co-sponsors; 15 of them Democrats and only 1 Republican. http://beta.congress.gov/bill/113th/house-bill/499/cosponsor...