The contribution reported Tuesday by The Sacramento Bee is the single biggest donation from an individual other than Oakland medical marijuana entrepreneur Richard Lee, the main sponsor of Proposition 19.
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Soros, a high-profile liberal, was one of the top financial backers of the 1996 measure that made California the first state in the nation to legalize marijuana for medical use.
The $1 million comes a day after the Yes on 19 campaign launched its first television ad.
Until now, neither side has seen a huge outpouring of cash, though supporters have significantly out-raised opponents.