For more than a year, the darlings of the Democratic left, Sen. Bernie Sanders of Vermont and Sen. Elizabeth Warren of Massachusetts, coexisted — just like those multifaith bumper stickers counsel from the backs of countless Subarus. But Utopia ain’t big enough for the both of them. The party’s presidential nomination is not a toy that can be shared. Sooner or later, they were going to have to duke it out.

That explains why they are fighting now, less than three weeks before the primaries begin. More illuminating is how they are fighting — the specific content of their contretemps. They’re battling over signals, not substance. That’s frequently the case in contemporary politics, and it’s a problem.

To review: Someone leaked a script apparently prepared for Sanders campaign volunteers in Iowa. They were instructed to say, when speaking to likely Warren voters, that “the people who support her are highly educated, more affluent people who are going to show up and vote Democratic no matter what.” In other words, “she’s bringing no new bases into the Democratic Party.”

Pretty mild stuff, as attacks go. What candidate ever lost a nomination by being too popular with the people who show up at the polls? In fact, if I had to bet, I would wager that the source of the leak was somehow linked to Warren’s own camp, because she and her team leaped on the script as an excuse to escalate. Warren — whose summer surge has waned in winter, leaving her in urgent need of headlines — accused Sanders of “trashing” her. And then, lo and behold, several anonymous sources delivered a counterpunch by leaking a previously undisclosed detail from a 2018 private meeting between the two progressive rivals. During the parley, the sources said, Sanders expressed doubts that a woman could beat President Trump in 2020.

Sanders furiously denied the report — Warren’s staff was “lying,” his verb of choice — but Warren administered the next jab on the record. Nope, it’s true, she averred. Sanders said it.

Even before the matter could be rehashed at Tuesday night’s debate, there was much to learn from this back-and-forth. First, consider the weird intensity of Sanders’s response. He had a lot of rhetorical space available, and yet he went straight to the incendiary “lying.”

Why? It was common in the post-Hillary moment to hear Democrats musing about the electability of a woman. Something had to explain Clinton’s stunning defeat in the electoral college, and many dedicated feminists believed the root cause was sexism. If Sanders had expressed the same opinion, he would have had plenty of company among some very pro-woman political analysts.

To be clear, I don’t agree. I think a better female candidate than Clinton, running a more focused and positive campaign, could have won in 2016 and can win in 2020. But that’s not the point. What matters is that Sanders seemed to have room to blame a misunderstanding or misinterpretation. He could have clarified; instead, he flatly denied. And thus, he put himself in the position of appearing to call Warren a liar.

At 78, and after a lifetime in politics, Sanders must have had a reason for going nuclear. That reason, I think, is that he understands his own base and how important it is to them that the proper signals be sent. The battle for the left is not about results. It’s about wishes.

You see this up and down the Sanders agenda. His Green New Deal proposals make vast promises on tight timelines that far exceed available technology. His supporters approve precisely because it is excessive. The excess shows how deeply he cares.

Or take Medicare-for-all. Sanders pledges an immediate transition from the current medical economy to a new one, and criticizes Warren’s proposal to make the transition over a period of years. Whether such enormous change is achievable with a snap of the fingers is beside the point; what matters to Team Sanders is the determination signaled by occupying the most extreme available position.

Given this mind-set, the danger to Sanders of admitting to a pragmatic discussion of sexism in politics becomes clear. It’s off-brand. His supporters want to hear him speak about what ought to happen in a perfect world, not what’s likely or practical in ours. A supporter of women should declare that a woman can win the White House — whether he believes it as a practical matter or not. Conversely, to say it can’t happen due to residual sexism is a sign of not wanting it enough.

It’s also revealing that Warren’s team saved this conversation to be deployed when the fight for the left finally heated up. She can’t go after Sanders on substance, because the same loose linkage to reality that characterizes his agenda is shot through her plans, too. Warren can’t criticize Sanders’s pie-in-the-sky while busily building castles in the air.

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