You are misinterpreting Lenin's work enormously. What Lenin is describing is fronting working class parties in bourgeois elections, like what PSL is doing, not working within bourgeois parties in bourgeois elections. This is abundantly clear if you read Lenin, Lenin's central thesis is that the working classes need their own, united party, and that said party needs to combine legal and illegal work to grow. The DNC is not a communist, socialist, or even vaguely anti-imperialist organization, it's an imperialist party representing capital alone.