Congress’s two loudest socialists are teaming up to take on credit-card companies—while applying their familiar analytical skills to the economics. Sen. Bernie Sanders and Rep. Alexandria Ocasio-Cortez last week proposed capping interest rates on consumer loans at 15%.

“There is no justifiable reason that a person—no matter their background—should be charged an interest rate higher than 15%,” AOC says. The pair argue that today’s median rate on credit cards, 21%, is too high given that banks can borrow from the Federal Reserve at less than 2.5%.

This is misleading, since credit cards are primarily payment networks. They process countless $5 transactions, send monthly bills, offer 24-hour customer service, scan for fraud, resolve disputes with merchants, and more. The cost of daily operations is high. Yet swiping is free for millions of Americans who pay their balances each month.

Price ceilings on any good or service inevitably reduce supply, and credit is no exception. Good luck trying to get a Visa or Mastercard under this plan, unless your credit score is stellar. Annual fees would return for basic cards. Recall what happened after Democrats in 2010 capped debit-card “swipe fees” on merchants. Banks quit offering free checking and raised monthly fees for millions of nonrich customers.

Bernie and AOC want to kill payday lending in particular. This would simply push people who need short-term cash toward pawnbrokers or organized crime. “Today’s modern-day loan sharks are no longer lurking on street corners breaking kneecaps to collect their payments,” Mr. Sanders says. “They wear three-piece suits and work on Wall Street.” It never occurs to him that the availability of legal loans is what helped to put Louie Legbreaker out of business.