Mr. Bernstein stipulated that while debt financing has its place, the White House also believes it has firm limits within its agenda. “The outcome of all this is going to be some mix of progressively raised revenues and investments in essential public goods with a high return financed by some borrowing.”

Looking Ahead, and to the Past

What would have to happen for these rock-bottom borrowing costs to rise significantly? There could be a crisis of confidence in Fed policy, a geopolitical crisis or steep increases in the Fed’s key interest rates in an attempt to kill off inflation. In a more easily imagined situation, some believe that if inflation remains near its current levels into the second half of the year, bond buyers may lose patience and reduce purchases until yields are more in tune with rising prices.

The resulting higher interest payments on debt would force budget cuts, said Marc Goldwein, the senior policy director at the Committee for a Responsible Federal Budget. Mr. Goldwein’s organization, which pushes for balanced budgets, estimated that even under this past year’s low rates, the federal government would spend over $300 billion on interest payments — more than its individual outlays on food stamps, housing, disability insurance, science, education or technology.

Last month, Brian Riedl, a senior fellow at the right-leaning Manhattan Institute, published a paper titled “How Higher Interest Rates Could Push Washington Toward a Federal Debt Crisis.” It concludes that “debt is already projected to grow to unsustainable levels even before any new proposals are enacted.”

The offsetting global and demographic trends that have been pushing rates down, Mr. Reidl writes, are an “accidental, and possibly temporary, subsidy to heavy-borrowing federal lawmakers.” Assuming that those trends will endure, he said, would be like becoming a self-satisfied football team that “managed to improve its overall win-loss record over several seasons — despite a rapidly worsening defense — because its offense kept improving enough to barely outscore its opponents.”

But at least one historical trend suggests that rates will remain tame: an overall decline in real interest rates worldwide dating back six centuries.

A paper published in 2020 by the Bank of England and written by Paul Schmelzing, a postdoctoral research associate at the Yale School of Management, found that as political and financial systems have globalized, innovated and matured, defaults among the safest borrowers — strong governments — have continuously declined. According to his paper, one ramification may be that “irrespective of particular monetary and fiscal responses, real rates could soon enter permanently negative territory,” yielding less than the rate of inflation.