Fight all you want, by all means, bring those in power further left. But at the ballot box come election day, I don't care if the dem wants to sacrifice puppies on weekends. Guaranteed their Republican opponent has a factory grinding puppies into various consumer products by the millions. And their 3rd party counterpart likely has a greater chance of getting elected to mayor of flavortown than to congress/president (and also still probably kills puppies casually among friends themselves).
Is this system bullshit? Yup. Is it the one we got? Yup. We need to deal within that reality. By all means, let's work to change that system, but for the love of God, let's not shoot ourselves in the foot when our favorite candidate doesn't stand a chance in hell, or the more likely candidate is blatantly flawed (but still better than the alternative)