If you want leftists to vote for dems, despite dems pissing on leftists at every possible chance and yelling at leftists to fall in line, I'll show you how.
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Point out that voting will never, ever, ever move the democrat party to the left. You cannot vote the party harder to the left.
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Point out that Republicans are going to remain fascists.
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Point out that voting third party is a spoiler vote and will result in fascists winning.
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Point out that the actual way to move to the left is to unionize and organize at the grassroots level, to apply bottom-up pressure on the top.
The answer is not to pretend that Biden is anything other than a Neoliberal Capitalist. Leftists will correctly point out that Biden is still a lukewarm neoliberal maintaining the status quo, and feel further alienated by being told they should love him anyways. That just encourages voter apathy.