What motive would Biden have to “evolve” on marijuana?

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As has been pointed out, Biden publicly supports decriminalization. The issue seems to be more that he's not talking about it, and for good reason, his stance on marijuana is not going to be a deciding factor for hardly any voters. Therefore it's not worth the time and energy of his campaign to dwell on the subject when there are issues that voters are far more concerned with and could influence their vote.

Decriminalization is as brave as being for civil unions in 2020.