The path of interest rates over the next year doesn’t just affect the cost of mortgages and car loans. It also makes it more expensive for the federal government to borrow.

And in recent years, that borrowing has become very expensive.

Interest payments on government debt have risen from 1.2 percent of gross domestic product in 2015 to an estimated 3.2 percent in 2025, according to the Congressional Budget Office, topping the previous highs from the 1980s. That’s $952 billion — more than the budget of the Department of Defense.

According to the nonpartisan budget scorekeeper, interest payments are forecast to continue rising, to 4.1 percent of G.D.P. in 2035. And that is not accounting for congressional Republicans’ plans to extend the 2017 tax cuts scheduled to expire at the end of this year, which would add $3.7 trillion to the deficit over 10 years unless offset by spending cuts.

If investors start to worry about whether the U.S. government can make good on its debts, that could push the Treasury to offer higher rates on Treasury bonds, creating a debt spiral.

Jared Bernstein, who served as chair of the White House Council of Economic Advisers in the Biden administration, said that the rising cost of debt service was a cause for concern.

“It’s not flashing red now, but it wouldn’t take much to get it there,” Dr. Bernstein said, noting expectations for lower revenues, higher spending and the risk of higher interest rates.