Those rates, in turn, should make it manageable for the government to finance a large national debt.

What really matters for the sustainability of public finances is not the size of the debt, but the ratio of debt service costs to the overall economy. Mr. Powell is effectively pledging to keep the numerator low (through low interest rates), while asking Congress to do what it can to keep the denominator high (by preventing a sustained economic collapse).

It’s a 180-degree inversion of a traditional relationship — between elected officials and independent central bankers — that economics students learn in school.

Politicians, in this conventional logic, always have an eye toward the next election. So they have every incentive to embrace economic policies that make people feel prosperous in the short run but could be damaging in the long term, such as by causing inflation or by risking a fiscal crisis. In such a case, you’d want an independent, hardheaded central banker focused on a longer time horizon and the power to rein in those tendencies, typically by raising interest rates if the economy is overheating.

But right now, the Fed is pulling out all the stops to shore up the economy while Congress is starting to show signs of bailout fatigue. Republican leaders have begun raising alarms about rising deficits. They’ve expressed opposition to federal help for state and local governments, while some Democrats have voiced concern that programs meant to support businesses don’t have enough strings attached, such as requirements to suspend dividend payments to shareholders or eliminate bonuses to top executives.

Instead, the hardheaded central banker is the one encouraging Congress to spend whatever it takes to get the economy through this crisis. It would be as if a doctor who normally implored a patient to lay off the spaghetti carbonara and milkshakes instead concluded that the patient was dangerously underweight and needed all the calories he could get.

This is not the first inversion of the usual relationship between elected officials and a central banker. After the 2008 financial crisis, Ben Bernanke, then the Fed chairman, often encouraged Congress to take more aggressive fiscal action. And the European Central Bank president, Mario Draghi, spent much of his recently concluded eight-year term doing the same.