Monetary policy depends on fiscal policy in an era of large debts and deficits. Suppose that the Fed raises interest rates to 5% over the next few years. This is a reversion to normal, not a big tightening. Yet with $18 trillion of debt outstanding, the federal government will have to pay $900 billion more in annual interest. Will Congress and the public really agree to spend $900 billion a year for monetary tightening? Or will Congress simply command the Fed to keep down interest payments, as it did after World War II, reasoning that "Fed independence" isn't worth that huge sum of money?
This additional expenditure would double the deficit, which tempts a tipping point. Bond markets can accept fairly big temporary deficits without charging higher interest rates—buyers understand that bigger deficits for a few years can be made up by slightly larger tax revenues or spending cuts over decades to follow. But once markets sense that deficits may be unsustainable, and that bond buyers may face default, restructuring or inflation, they will demand still-higher interest rates. Higher rates mean higher deficits—leading to a fiscal death spiral.