What did Donald Trump do today?

He thought the United States' federal debt was like a mortgage.

While on trial in the Senate, Trump is attending the World Economic Forum in Davos, Switzerland. Today, amidst billionaires and Nobel Prize-winning economists, he said this:

TRUMP: And one of the reasons I'd like to see the interest rates lowered, frankly, is because I'd like to refinance the debt and pay off the debt.

U.S. federal debt cannot be refinanced.

Trump is confusing bonds issued by the Treasury with callable debt. This is debt that, like many mortgages, can be paid back early or renegotiated. When interest rates go down, homeowners can use that ability to repay in advance—with money borrowed at the new rate, if necessary—to negotiate with banks.

But federal debt is not callable. It's a legal obligation to pay a fixed rate over a fixed period of time to whoever owns it at any given moment. Even if interest rates fall to zero, existing bonds (which may not mature for up to 30 years) issued at 2% or 3% cannot be replaced with bonds paying 0%. Instead, the government would have to overpay by repurchasing the higher-rate bonds.

The only way that the United States could pay less than its legal obligation would be to default, which would have immediate and catastrophic consequences for the world economy. (Trump, whose companies have failed to pay their debts many times, has threatened to do exactly that with the United States' credit.)

The real reason that Trump wants the Federal Reserve to slash interest rates because low rates flood the economy with money, at the cost of making future recessions harder to avoid.  (It also makes him personally richer, since he is carrying enormous amounts of callable debt, meaning he personally can refinance it and save money.)

That said, it's not the first time Trump has made this elementary mistake. He appears to genuinely believe that a Treasury bond and a bank mortgage are the same thing—or, at least, he seems to be gambling that his supporters will not know the difference.

Why is this a problem?

• The United States economy is too important for a president to be making this kind of simple mistake over and over again.

• Presidents should be willing and able to put the country's financial interests ahead of their own.