‘IF THE Washington Post wants to say that our ideas are bold, I accept that,” Sen. Bernie Sanders (I-Vt.) said Thursday in response to a critical editorial we ran about him. “We’ve got to create an economy that works for the middle class. And whether The Washington Post likes it or not, that’s what I intend to do.”

In fact, we would love that — and we were heartened that Mr. Sanders chose to engage with our editorial. Yet our disagreements with Mr. Sanders are not as he portrayed them; they do not concern the problems he chooses to address or the boldness with which he proposes to address them. The nation could use big measures to take on climate change, advance public health, tackle poverty, shore up entitlement programs, boost education, improve our democracy and do all sorts of other things that Mr. Sanders cares about. We argue for policies such as a carbon tax and public campaign financing, even though they are subject to massive and possibly insurmountable political opposition, because they would lead to large and needed changes.

What concerns us is not that Mr. Sanders’s program to tackle these issues is “radical,” as he put it, but that it is not very well thought out. We are far from the only ones, for example, to point out that his health-care plan rests on unbelievable assumptions about how much he could slash health-care costs without affecting the care ordinary Americans receive. “Their savings numbers are — well, politely said — simply wrong,” Emory University health-care expert Kenneth E. Thorpe told Vox. Mr. Thorpe, who is not hostile to single-payer systems of the type Mr. Sanders favors and has even advanced single-payer plans of his own, released an analysis Wednesday finding that Mr. Sanders’s proposal would cost $1 trillion more than the candidate estimated. That is not over a 10-year budget window. That is every year.

Mr. Sanders’s response to concerns over health-care costs was that other countries, such as Canada and France, spend much less than the United States per person on health care. That is true, but the question is how, specifically, he would make the model work here. The countries he praises ration care in ways that federal health programs in the United States, such as Medicare, do not. While there may be a fair case for a single-payer health-care system, Mr. Sanders does not make it. Instead, he promises comprehensive benefits without seriously discussing the inevitable trade-offs. That is not just bold; it is half-baked.

Health-care policy is only one place where Mr. Sanders makes solving the country’s difficult problems seem easy and obvious when reality is messier. He would use higher taxes on Wall Street and the rich to fund vast new programs, such as free college for all, but has no plausible plan for plugging looming deficits as the population ages. His solution to the complex international crises the United States must manage is to hand them off to others — though there is no such cavalry. This might not distinguish him much from other politicians. And that is part of the point: His campaign isn’t so much based on a new vision as on that old tactic known as overpromising.