Net Interest is the fastest-growing line in the federal budget — about $881 billion in FY2024, up from roughly $350B just a few years ago. It's not funding a program or agency. It's the interest the federal government pays on Treasury debt held by the public — bonds, notes, and bills sold to investors (both domestic and foreign).
The growth comes from two compounding factors: total federal debt has grown from about $20T to over $35T, and interest rates have moved sharply higher since 2022. Net Interest already exceeds federal spending on defense, Medicare general-fund subsidies, and most discretionary categories. Unlike other lines, this one is structurally locked in by past borrowing decisions and cannot be changed without either paying down principal or restructuring the debt itself.