Sen. Elizabeth Warren is campaigning for a 2% surtax on anyone’s net worth over $50 million and 6% on more than $1 billion. Most discussion of the wealth tax has focused on whether it could generate sufficient revenue to fund Ms. Warren’s various spending plans, and the revenue likely would be far less than she projects. But that focus overlooks the ways the massive new tax would harm all Americans.

If you take comfort in Ms. Warren’s claim that her tax would apply only to the wealthiest Americans, think again. In 1913 the...