Failing to do any of these things would be a default on the president's duty to “take care that the laws be faithfully executed.” The president may not be able to obey all three sources of law; if so, Aaron argues, he should make the payments and ignore the debt ceiling. “The debt ceiling is the fiscal equivalent of the human appendix — a law with no discoverable purpose,” he writes. “If Congress leaves the debt ceiling at a level inconsistent with duly enacted spending and tax laws, the president has no choice but to ignore it.”

Aaron's argument echoes the elegant analysis last fall by law professors Neil Buchanan of George Washington University and Michael Dorf of Cornell. These two prominent scholars concluded that paying appropriated monies and interest on the debt represents the “least unconstitutional” option open to a president when Congress refuses to approve a debt-ceiling increase.

In the same vein is a 2011 essay by Peter Shane of the Ohio State University — one of the most careful students of presidential authority writing today. “The legal path is not clearly marked either way,” he writes. But Shane notes that both paying the debt and the appropriated monies, and impounding the monies and defaulting on the debt, involve a president violating some legal command.

All the commands can't be right.

A default on the debt poses an existential threat to both the nation and the world economy. Experts I trust worry that it could precipitate a new 1929-style global depression. It would be in the economic sphere the rough equivalent of a sudden attack on the United States. No one would question that a commander-in-chief could respond to such an attack — even if Congress refused to act in the face of the threat. In that situation, the members of Congress, not the president, would be in violation of their oaths.

If a president sits by and allows the world economy to collapse, that president might be held to have violated his oath. If a president refuses to sit by, the same charge can be made. In either case, the question could only be resolved by impeachment proceedings. After that, we will know who has the power, for good or ill.

If I were president, I think, I'd rather be impeached for acting than for standing by. Obama admires Abraham Lincoln. On July 4, 1861, Lincoln defended his refusal to obey a writ of habeas corpus during the secession crisis: “Are all the laws, but one, to go unexecuted, and the government itself go to pieces, lest that one be violated?”