Ms. O’Donnell: “You’re going to propose a plan to the American people, and you’re not going to tell them how much it costs?”

Mr. Sanders: “Of course, I will. Do you know exactly what health care costs will be, one minute, in the next ten years if we do nothing? It will be a lot more expensive than a Medicare for all single-payer system.”

— in an interview with CBS in January

This is disputed. Mr. Sanders has a point that a large degree of uncertainty exists in projecting how much a Medicare-for-all system would cost, though he has also been comfortable offering his own estimates in the past. And his claim that a single-payer health care system would be less expensive than the status quo is disputed by some health economists.

Mr. Sanders said in July that Medicare for all would cost “somewhere between” $30 trillion and $40 trillion over a ten-year period. That is in line with estimates from several major studies: $34 trillion over a decade, according to the liberal Urban Institute; $32.6 trillion, according to the conservative Mercatus Center; and $24.7 trillion, according Kenneth E. Thorpe, a health policy economist at Emory University. (Lower estimates include $13.5 trillion from a study from the University of Massachusetts Amherst and $9.6 trillion from Gerald Friedman, a health economist at the same university who did not contribute to that study.)

These estimates vary so widely because many key details of Medicare for all remain unsettled, including how doctors and hospitals would be compensated and how prescription drug prices and administrative fees would change. Other effects, like whether a single-payer system would lead to people using more health care services, are difficult to predict precisely. These myriad factors are why the Congressional Budget Office previously declined to estimate the cost of Medicare for all.

Just as it is difficult to put a price tag on a single-payer system, it is similarly difficult to say with certainty that it will cost less than the current health care system, despite Mr. Sanders’ assertion that it will. Some analyses back his argument while others contradict it.

The Urban Institute found that total health spending would reach $52 trillion from 2020 to 2029 under existing law compared to $59 trillion under Medicare for all. The RAND Corporation, a nonpartisan research group, estimated the current system costing $3.8 trillion in 2019 versus $3.9 trillion if a single-payer system had been in place.