Elizabeth Warren has been justly criticized for the magical math in her Medicare for All proposal. That health plan would cost perhaps $34 trillion, according to outside estimates. But Ms. Warren says the true figure is only—alakazam!—$20.5 trillion.

Her wealth tax is the same way, as a fiscal fact check this week demonstrates. Ms. Warren wants to tax the net worth of affluent Americans 2% a year on assets above $50 million, and 6% on billionaires. She says it would raise $3.75 trillion over 10 years.

Her...