Interest payments on the federal debt last year equaled 1.4 percent of the nation’s annual economic output, according to the Congressional Budget Office. That leaves less money for other things. In 10 years, the budget office projects that interest payments will increase to 3 percent of the nation’s annual economic output.

Those payments once flowed almost exclusively to Americans, but foreign lenders now hold about a fifth of the debt, so the interest is drained from the economy.

And the problem could get worse, quickly. Since the financial crisis, interest rates have remained at very low levels. Investors have lined up to lend money to the federal government. Sometimes they have even agreed to pay for the privilege. But stronger economic growth could create more alternatives for investors, and that could force the United States to compete by offering higher interest rates.