Sen. Kamala D. Harris (D-Calif.) launches her 2020 presidential campaign in Oakland, Calif., on Jan. 27. (Elijah Nouvelage/Reuters)

SEN. KAMALA D. HARRIS (D-Calif.), a freshly minted presidential candidate, sketched an appealing picture of Medicare-for-all at a CNN town hall on Monday. Yes, she said, the plan would require doing away with insurance companies. But, she argued, who would miss them? “Who of us has not had that situation where you’ve got to wait for approval and the doctor says, ‘Well, I don’t know if your insurance company is going to cover this’? Let’s eliminate all of that, let’s move on.”

Actually, no one can really eliminate “all of that” — not Ms. Harris and not possible 2020 candidate Sen. Bernie Sanders (I-Vt.). Even if the United States adopted Medicare-for-all or some other version of national health insurance, Americans would not get everything they want whenever they want it. No one, in any country, does.

Ms. Harris and other Medicare-for-all advocates expect that the country would save money and hassle if the government, not insurance companies, paid for Americans’ health care. Insurance company profits and marketing costs could be redirected to health care. Whether the savings would be as large as expected; whether the government could manage things more efficiently; whether Americans really want to disrupt the existing system — all of those are legitimate questions for debate.

But what Medicare-for-all could not do — and what Ms. Harris and others who may tout the idea during the coming campaign cannot claim honestly — is end health-care rationing. Doctors, or somebody, would still have to tell patients that some procedures or prescription drugs are covered and some are not. The government would have to decide what does get covered. Some people would still be upset, convinced that federal bureaucrats denied them life-changing or life-saving treatment.

A spokesman for Ms. Harris argued to us that at least those federal bureaucrats would not be motivated to restrain costs in order to maximize profits, as insurance companies are, which might lead to different choices about what gets paid for. Perhaps, but the fact remains that the government would have to manage costs, which would require hard-nosed rationing, or else the already daunting challenge of paying for single-payer would become even more so. Government health-care systems in the European social democracies, the models that supposedly prove that Medicare-for-all can work as described, are in fact far less generous than the proposed Sanders-Harris plan.

If the nation were building a health-care system from scratch, single-payer might be the rational choice. Even now, with many Americans reasonably satisfied with their employer-sponsored coverage, politicians can make an argument that they’d be better off in a different system. But they should not make that argument by exaggerating the benefits or lowballing the costs of single-payer, as Medicare-for-all advocates so often do. Any system will demand tradeoffs and constraints.