To put it another way: Can you think of one way that Bernie Sanders is signaling respect to voters outside of his base?

He has taken a nearly maximalist liberal position on every major issue. It’s especially striking from him, because he has shown over his career that he grasps the importance of building a coalition.

Sanders once won over blue-collar Vermonters with help from a moderate position on guns. “We need a sensible debate about gun control which overcomes the cultural divide that exists in this country,” he said in 2015, “and I think I can play an important role in this.” He was also once an heir to organized labor’s skepticism of large-scale immigration. “At a time when the middle class is shrinking, the last thing we need is to bring over in a period of years, millions of people into this country who are prepared to lower wages for American workers,” he said in 2007.

Now, though, Sanders has evidently decided that progressives will no longer accept impurities — or even much tactical vagueness. He, along with Elizabeth Warren, has embraced policies that are popular on the left and nowhere else: a ban on fracking; the decriminalization of border crossings; the provision of federal health benefits to undocumented immigrants; the elimination of private health insurance.

For many progressives, each of these issues has become a moral litmus test. Any restriction of immigration is considered a denial of human rights. Any compromise on guns or health care is an acceptance of preventable deaths.

And I understand the progressive arguments on these issues. But turning every compromise into an existential moral failing is not a smart way to practice politics. It comforts the persuaded while alienating the persuadable.

F.D.R. and Reagan understood this, as did Abraham Lincoln and many great social reformers, including Frederick Douglass, Jane Addams, Martin Luther King Jr. and Cesar Chavez. Strong political movements can accept impurity on individual issues in the service of a larger goal: winning.