GO TO ANY American high school holding an election for class president and inevitably one candidate will craft an agenda of alluring promises—free pizza at lunch, limitless recess after—well beyond their capacity to actually turn them into reality. The same impulse animates the latest economic-policy speeches delivered by Kamala Harris and Donald Trump in battleground states. Their competing visions offer utopia without trade-offs—economics without the economising—in which spending is higher, taxes are lower, deficits are unimportant, inflation is licked, jobs are protected and growth is high.