Net interest on the federal debt

Interest paid on the federal debt, net of interest received. It is mandatory spending: it flows from the existing stock of debt and the rates that debt was issued at, and no annual vote adjusts it.

Two forces drive it, and they can move independently. The stock of debt grows with every deficit. The average rate paid on it changes slowly, as older securities mature and are refinanced at whatever rates prevail. A period of rising rates therefore shows up in this series gradually, over years, rather than at once.