In its traditional form, a single-payer health care system would effectively outlaw private health insurance as we know it. The Medicare for All plan backed by Sen. Bernie Sanders (I-Vt.), for example, would end today's private health insurance market in a period of four years, forcing nearly 180 million Americans off of their existing plans in the process.

To the plan's most ardent backers, this is an objectively positive development. After Sen. Kamala Harris, who supports the Sanders plan, said at a presidential town hall Monday night that she favors eliminating all private health insurance, even for people who like their plans, a policy staffer for Rep. Alexandria Ocasio-Cortez (D-N.Y.) tweeted, "Yes, we're going to get rid of the entire health insurance industry. That's a feature, not a bug."

But as Harris appears to have discovered, most people don't see it that way. There is even resistance within her own party. In the 24 hours following her remarks, a number of prominent Democrats distanced themselves from the idea, including Sens. Dick Durbin (Illinois), Tim Kaine (Virginia), and even Harris' fellow senator from California, Dianne Feinstein, with Feinstein saying, "Well, I'm not there."

Harris, it seems, is not quite there anymore either; or if she is, she is also somewhere else. Last night, she gently moderated her position, with a spokesperson telling CNN that she is open to other policy paths, although she continues to support a single-payer plan that would end private health insurance as well.

It is not exactly a walkback, but it is a tacit acknowledgment of the resistance to her initial remarks. She continues to support a plan that would make today's private health insurance plans illegal while forcing most everyone onto a government-run insurance system. But she supports alternatives as well, presumably ideas like creating a government-run insurance plan that would be sold alongside private plans, or allowing more people to buy into the existing Medicare system, or something like it.

In other words, she also supports plans that are not full-fledged single-payer, the entire point of which is to replace all existing insurance with, yes, a single government-run health coverage plan.

What Harris encountered was the obstacle that has bedeviled health care reformers on both the left and the right for decades: Although public satisfaction with the health care system writ large is often fairly low, polls consistently find that a majority of people like their own health insurance plans and doctors, and they recoil from plans that would cause them to lose their existing coverage arrangements.

That dynamic is what helped kill a planned health care overhaul under President Bill Clinton, and it is why President Barack Obama sold the Affordable Care Act on the false promise that it would not cause anyone to lose their existing health insurance coverage or doctor. It is also one of the reasons that the Republican effort to repeal Obamacare failed, and it remains a major impediment to overhauling Medicare. Similarly, recent surveys find that Medicare for All is only popular until people are told that it would eliminate private health insurance.

When it comes to health care, the public really, really, really does not like disruption. But the entire point of single-payer, which is to say the entire point of Sanders-style Medicare for All, is disruption on a massive scale. All of the other problems—the massive increase in federal spending, the administrative complexity, the job loss, and the medical provider reimbursement cuts—are in some sense secondary.

The incredible unpopularity of any plan that openly proposes to upend current coverage for tens of millions of people is a political barrier no one has managed to overcome. That is why Democrats have typically avoided advertising that their plans would do so, and why some are attempting to brand ideas that are not full-fledged single payer as Medicare for All.

Medicare for All is popular as a slogan, but much of its popularity stems from the ambiguity surrounding what, exactly, it means. That ambiguity can persist for a while, but it is harder to sustain when the plan is put front and center in a major presidential campaign. By foregrounding single-payer's disruptive effects at the beginning of her presidential campaign, Harris provided as succinct a demonstration as you are likely to see of why, for the foreseeable future, Sanders-style Medicare for All is all but certain to fail.