On the campaign trail, President Donald Trump promised to cap credit card interest rates at 10 percent. But since his inauguration, the federal agency that regulates banks has abandoned its support for a state law designed to reduce credit card interest rates, which now average 24 percent and cost American consumers $120 billion every year.

At issue is a 1978 Supreme Court ruling permitting banks to export their home state’s light-touch lending regulations when doing business in states with stricter laws, such as interest rate caps. Since then, credit card companies and big banks have moved their headquarters to Delaware, South Dakota, and Utah — which have no state interest rate caps.

But after the Supreme Court decision, Congress carved out an option for states to set their own interest-rate caps on all banks doing business within their borders. Iowa and Puerto Rico have successfully done so, capping interest rates at 32 percent.