The U.S. Constitution gives Congress the power to write laws regulating commerce and financial markets — including interest-rate limits on credit cards. Trump has called for a one-year cap at 10%, but no such law currently exists, and a presidential proclamation alone doesn’t change private-sector interest rates.

U.S. President Donald Trump ​on Thursday said he does not like the idea of letting people use ‌401 (k) retirement funds for a down payment on a house, even though it was floated by his own chief econo...