“The basic idea that if you issue too much debt you will crowd out large amounts of valuable private capital investment was a reasonable view in the early to mid-1990s,” Mr. Summers, a Harvard economist, said in an interview. With very low interest rates worldwide, it isn’t today, he said.

Mr. Summers and other non-alarmist economists are not suggesting that debt never matters.

They say that in this environment — with low interest rates to the horizon, no evident inflation pressures, and with worthwhile policies that could enhance the nation’s long-term outlook — it’s foolish not to borrow money.

Mr. Blanchard, the former I.M.F. chief economist, emphasizes that interest rates are comfortably below the rate at which the economy is growing. That means that, despite high debt levels in the United States, it shouldn’t matter if the nation keeps borrowing money because its capacity to pay is growing faster than interest costs.