Mr. Sanders has described his health care plan as “Medicare for All,” but it would be more generous, giving Americans broader coverage without premiums, deductibles or co-payments. It would replace not only Medicare but also Medicaid and the Children’s Health Insurance Program. A table in his economic adviser’s analysis shows that all public spending currently going to military, veterans’, American Indian and other health programs would be part of the financing for his single-payer plan, yet Mr. Gunnels said veterans’ and American Indian health programs would remain intact. That suggests double-counting, or financing the existing programs while claiming the sums to offset the single-payer plan. He did not address military benefits in an email exchange.

The critics — many of whom support the concept of single-payer plans, including Paul Krugman, the Nobel Prize-winning economist and Op-Ed columnist for The New York Times — note the difficulty that Mr. Obama has had in winning and putting into effect his less-ambitious law, which keeps the private insurance and health care sectors in place. They worry that Mr. Sanders, as president, would exhaust his political capital on what they call a fool’s errand, at the expense of other initiatives on education, infrastructure, climate change, worker benefits — and the Affordable Care Act itself.

“The single-payer idea has enormous appeal: coverage for everyone, some effort to use the government’s bargaining power to hold down overall costs, clean out the godawful administrative mess that the U.S. health care system is and save money there,” said Henry J. Aaron, a longtime health economist at the Brookings Institution in Washington.

But he called it a “fairy tale” in this political climate. Along with other economists in a “lefty chat group” he joins online, Mr. Aaron said, he believes that if Mr. Sanders were elected and fought for a single-payer plan, it “would rapidly destroy his administration by using up every ounce of political capital he’s got.”

On his campaign website, Mr. Sanders proposes more than $18 trillion in new spending over 10 years; he does not account for some ideas he favors, like universal prekindergarten and child care, that could put the total above $20 trillion. About $14 trillion of the total is for health care; the rest is chiefly for infrastructure, free college, Social Security, paid family leave and clean-energy initiatives.

Adding $20 trillion to projected federal spending would mean about a 37 percent increase in spending through the 2026 fiscal year — close to the 40 percent that Mrs. Clinton suggested. But Kenneth E. Thorpe, a prominent health policy economist at Emory University who advised the Clintons in the 1990s, recently concluded that Mr. Sanders’s health plan would cost $27 trillion, not $14 trillion, which would put total spending for all of his initiatives above $30 trillion through 2026.