Deep Dive: What Is the National Debt — How Government Borrowing Affects Bond Yields, Interest Rates, and Your Portfolio
The U.S. national debt surpassed $37.6 trillion in the third quarter of 2025, a figure so large it has become almost abstract. But behind that headline number lies a mechanism that directly shapes the interest rate on your mortgage, the yield on your bond portfolio, and the long-term trajectory of stock market valuations. Understanding the national debt is not just a matter of fiscal policy — it is an essential piece of any informed investment thesis. For investors, the national debt matters because the government finances itself by issuing Treasury securities — bills, notes, and bonds — that compete with every other fixed-income instrument for capital. When the Treasury needs to borrow more, it must offer competitive yields to attract buyers, and those yields ripple across the entire financial system. With federal net interest payments now running at an annualized rate of $1.23 trillion as of Q4 2025, servicing the debt has become the fastest-growing line item in the federal budget, raising questions about fiscal sustainability that markets are increasingly pricing into long-term bond yields. This guide breaks down how the national debt works, why debt-to-GDP matters more than the raw dollar figure, how Treasury issuance affects the bond market, and what it all means for investors building portfolios in an era of persistent fiscal deficits.