Finally, some conservative policy makers besides Mr. Mulvaney have convinced themselves that crashing into the debt ceiling won’t be a big deal because the government can “prioritize” its bill payments, so that interest on Treasury debt will be paid on a current basis, while other bills sit unpaid.

Understanding the false allure of prioritization requires a little background. Hitting the debt ceiling is not the same as a government shutdown or other fiscal brinkmanship. Think of the United States, acting through the Treasury, as holding a bank account at the Federal Reserve. Every day, millions of bills arrive and are promptly paid by debiting Treasury’s account at the Fed. At the same time, millions of dollars in tax and other receipts arrive and are credited to that bank account. The money coming in is systematically less than the money being disbursed (that’s what it means to run a deficit), and Treasury makes up the difference by borrowing in the capital markets.

A government shutdown occurs when the Treasury has money in its bank account but Congress refuses to appropriate the funds necessary for the government to function. Crashing into the debt ceiling, by contrast, would occur if Treasury had no money in its bank account because Congress prohibited it from funding deficits through incremental borrowing.

If Treasury hits the ceiling, it has only two realistic responses. Treasury can pay the government’s bills on a first-in, first-out basis, with the wait for payment growing every month, or it can prioritize bills, as Mr. Mulvaney and others have suggested it would.

But there are profound doubts as to whether the Treasury could even implement prioritization, beyond ring fencing interest payments, because its payment systems are designed to pay all claims as they are due, regardless of their origin. More important, prioritization is default by another name. The consequences are the same, regardless of which i.o.u.s Treasury chooses to dishonor.

All valid claims against the United States are backed by the credit of the United States, full stop; the Constitution does not contemplate that some claims are more senior than others. The deliberate nonpayment of billions of dollars of uncontested claims every month thus constitutes default, even if the Treasury is paying some of its other debts. The resulting class-action lawsuits will enrich generations of lawyers.

Once the unthinkable happens, no future constraints on congressional irresponsibility with regard to the national debt will remain. Prioritization will constitute the intentional subordination, not just of one claim to another, but of all claims to the pettiness of congressional politics. As a result, the once unassailable credit of the United States will become a perennial hostage to politics, and in response the debt markets will demand much higher interest rates.

These are noisy times in Washington. But even in this context, the awfulness of a debt ceiling crisis should galvanize us. Like an impending execution, it should concentrate our minds — now, while something can still be done.