In his live-streamed statement making the announcement, Sanders focused on “the ideological struggle” both within the Democratic Party and in the country as a whole — a struggle he insists that he has won. You can also understand it as a struggle to shift the balance between imagination and practicality.

Let’s look at what stood out as the heart of Sanders’s statement Wednesday:

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As many of you will recall, Nelson Mandela, one of the great freedom fighters in modern world history, famously said, and I quote, it always seems impossible until it is done, end quote. And what he meant by that is that the greatest obstacle to real social change has everything to do with the power of the corporate and political establishment to limit our vision as to what is possible and what we are entitled to as human beings. [...] It was not long ago that people considered these ideas radical and fringe. Today they are mainstream ideas and many of them are already being implemented in cities and states across the country. That is what we have accomplished together.

Like many radicals, Sanders spent much more time thinking about how to translate his principles into policy than he did on how to turn his policy proposals into law or implement them once they became law. I’ve been plenty critical of his tendency to wave away concerns about getting ambitious policy proposals passed, but he refused to let practicality limit his imagination.

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Indeed, for him that was vital: He believes that if you limit your ideas to what seems politically possible in the short term, you’ve already given in to the “establishment,” since they’re the ones who determine what’s politically possible. As long as you do that, profound change will never come to pass.

You might find that to be the only path to long-term success, or a recipe for failure. Either way, Biden is at the other end of the spectrum on this question. I wouldn’t be surprised if Biden would be perfectly happy with plenty of Sanders’s proposals, if you could just snap your fingers and make them happen.

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But like many in the Democratic Party, Biden starts from his understanding of the constraints on policy-making imposed by the makeup of Congress, the power of interest groups and current public opinion, then builds his proposals to fit within those constraints.

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That’s why the plans Biden has offered this year are significantly more liberal than what the Obama administration did — not because Biden is personally more liberal than Barack Obama, but because the policy space has expanded to make things such as a $15 an hour minimum wage more possible.

Sanders, of course, would say: “That’s because I made it possible by advocating for it when it was considered out of the mainstream!” Which is partly true (a lot of other people were involved, too). But where Biden has a good case to make is the difference between being a president and being an activist, or even being a presidential candidate.

As a candidate, Sanders could spend years pushing for something like single-payer health care without ever actually having to face a moment of success or failure. He could engage in that long slog of advocacy, always pushing ahead no matter how much resistance he got. But a president seeking to enact health-care reform has to have a bill, which will move through the legislative process and eventually get an up-or-down vote.

And if it fails, you can wind up waiting a long time to get another chance (it took 16 years between the failure of Bill Clinton’s reform plan and the passage of the Affordable Care Act). Some of Sanders’s most aggressive supporters argue that anyone who doesn’t support full single payer literally wants people to die in an inferior system, but you could just as easily argue that anyone who supports a health-care plan guaranteed to lose in Congress also wants people to die, if a less sweeping but still very beneficial reform might have had a chance of passing.

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Finding the right balance between these two impulses — determining what you try to do based on principle and imagination, or fitting your ambitions within the constraints of political reality — is one of the core challenges of governing.

People on the left still debate whether Obama found that balance — leftists say he was far too timid on many issues, while his defenders say that the leftists have a naive view of what he could have accomplished.

So this will be Sanders’s task if Biden is elected: to be the voice of imagination and push Biden not to be so quick to accept the constraints of the moment. It wasn’t in the cards for Sanders to become president, and maybe that’s for the best. He’s already shown that from the outside he can help expand the scope of the possible. We’re going to need him to keep doing it.

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