Mr. Sanders’s story continues with fantastical claims about how he would make the European social model work in the United States. He admits that he would have to raise taxes on the middle class in order to pay for his universal, Medicare-for-all health-care plan, and he promises massive savings on health-care costs that would translate into generous benefits for ordinary people, putting them well ahead, on net. But he does not adequately explain where those massive savings would come from. Getting rid of corporate advertising and overhead would only yield so much. Savings would also have to come from slashing payments to doctors and hospitals and denying benefits that people want.

He would be a braver truth-teller if he explained how he would go about rationing health care like European countries do. His program would be more grounded in reality if he addressed the fact of chronic slow growth in Europe and explained how he would update the 20th-century model of social democracy to accomplish its goals more efficiently. Instead, he promises large benefits and few drawbacks.

Meanwhile, when asked how Mr. Sanders would tackle future deficits, as he would already be raising taxes for health-care expansion and the rest of his program, his advisers claimed that more government spending “will result in higher growth, which will improve our fiscal situation.” This resembles Republican arguments that tax cuts will juice the economy and pay for themselves — and is equally fanciful.