A new tax proposal in Congress aims to stick it to the rich. But if passed, it could devastate the U.S. financial system and ruin the value of ordinary Americans’ retirement accounts.

The proposal, introduced by a team of Democrats in the House and Senate, would assess a penalty each time someone sells a stock, bond, or other financial instrument. It would tax each of the roughly 10 billion U.S. equity market trades each year, among other transactions.

The goal, presumably, is to hit the rich. But the stock market is not just a tool for the wealthy.

Some of the largest shareholders and beneficiaries of our modern financial system are pension funds for public-sector employees and private retirement account holders. Firefighters, teachers, university endowments, and private retirement savings all benefit from sophisticated equity markets. Many employers issue short-term debt to cover payroll and young start-ups sell securities to fund their growth.

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