You Are Viewtiful
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And Barack Obama was opposed to gay marriage when he was elected. Hillary is a smart politician. She is pragmatic. It's very likely that what she says publicly doesn't represent her actual views. But you have to understand that she is running for the Democratic party. She wants to run on issues that Southern Demoocrats wont run away from, issues that Dems in red states can run on without taking backlash. It isn't just about one person and their positions.

http://www.gallup.com/poll/179195/majority-continues-support-pot-legalization.aspx
Even in the South, depending on the poll, it's a 50/50 issue. Even then, what Sanders is proposing will not force Southern states to legalize marijuana, it will just give them the option to do so if they so please.
The amount of Democrats you energize by proposing legalization heavily outweighs the number of "shy Democrats" you scare away.