When President Donald Trump floated the idea of capping credit card interest rates, it probably sounded great to anyone carrying a balance. But JPMorgan Chase & Co. (JPM) isn't buying it, and the bank's CFO Jeremy Barnum has some thoughts about why this could backfire spectacularly.

A Recipe for Losing Access to Credit

During JPMorgan's fourth-quarter earnings call on Tuesday, Barnum didn't mince words about what he sees coming. "People will lose access to credit on a very, very extensive and broad basis," he said, warning that the move would likely impact the "people who need it the most."

The irony here is painful: a policy meant to help struggling borrowers could end up shutting them out of credit markets entirely. Barnum said such price controls would "dramatically" alter how credit is provided, fundamentally changing what he described as "among the most competitive businesses that we operate in."

He was refreshingly blunt about the impact on JPMorgan itself. "This is a big business for us," Barnum acknowledged, making it clear that with price controls in place, it might not be anymore. At the end of the fourth quarter, JPMorgan was sitting on $402 billion worth of consumer loans and credit card balances.

Policy by Social Media

When asked whether the administration had actually consulted banks before announcing this proposal, Barnum's answer was telling: "This is happening very quickly in a sort of unconventional way, starting with a social media post." Translation: no, they hadn't bothered to ask.

It's the kind of policy rollout that makes bank executives nervous, announcing major financial regulations via tweet without running it past the institutions that would have to implement it or the regulators who understand the knock-on effects.

Strong Earnings, Weak Stock Response

The credit card controversy overshadowed what were otherwise solid quarterly results. JPMorgan reported $46.8 billion in revenue for the fourth quarter, up 7% year-over-year and beating consensus estimates of $46.02 billion.

Earnings per share came in at $5.23, topping the $4.92 estimate. That included a $0.60 boost from the bank's acquisition of Apple Inc.'s (AAPL) credit card portfolio.

Despite the beat, investors weren't impressed. Shares dropped 4.19% on Tuesday, closing at $310.90, though they recovered slightly with a 0.48% gain in overnight trading.

The fundamental tension here is straightforward: if you cap the rates banks can charge on credit cards, they'll respond by becoming much more selective about who gets approved. Subprime borrowers with spotty credit histories are the first to get cut off, because banks can't charge enough to compensate for the higher risk of default. It's economics 101, and it's why price controls rarely work the way policymakers hope.