The idea of “Medicare for All” has energized progressives ahead of November’s midterm elections. Across the country, candidates like New York congressional hopeful Alexandria Ocasio-Cortez have rallied to the slogan. Vermont Sen. Bernie Sanders introduced the Medicare for All Act last year and has so far rounded up 16 co-sponsors. Last month, 70 House members formed a Medicare for All Caucus.

But there is an enormous gulf between the appealing idea of Medicare for All and the incredibly expensive reality. According to my...