AMONG THE many misconceptions fueling Donald Trump’s presidential campaign, one of the most stubborn, and most pernicious, is this: Government should run like a business, ergo Mr. Trump, a businessman, is especially qualified for the White House. As evidence that this is a dangerous fallacy, we would cite Mr. Trump’s CNBC interview Thursday, in which he mused about getting control over nearly $19 trillion in federal debt.

After rambling about how he is a “low-interest-rate person,” in part because cheap interest makes the federal debt manageable, which is true enough, he added that, as president, “I would borrow, knowing that if the economy crashed, you could make a deal.” He separately hypothesized that the United States “can buy back [debt] at discounts. You can do things with discounts.”

If these phrases mean anything, they contemplate at least partial repudiation of the U.S. government’s obligations, sacrosanct since the time of the founding. The “full faith and credit” of the United States, established over centuries and embodied in its debt, is the glue that holds global finances together. The minute the United States tried to reduce its debt load by offering creditors less than 100 percent of principal and interest — i.e., by “discount,” or “making a deal,” like Argentina or Greece — every institution that had taken this country at its word would be instantly destabilized.

Those institutions would include the Chinese government and others abroad, holders of a third of the debt — which might be okay with Mr. Trump. But they would also include the Federal Reserve, which holds 13 percent of U.S. debt, and the Social Security Trust Fund (which Mr. Trump has promised to protect), which, combined with other federal accounts, holds 28 percent. And don’t get us started about pension funds, money market mutual funds and community banks.

Another name for this debt-reduction method would be: a giant cramdown imposed on the whole world, including the American people. Note that it’s much more radical than a very questionable idea that got traction among Republicans during the last showdown over the federal debt limit: namely, paying interest and principal while stiffing, temporarily, other claimants such as vendors and contractors. That alternative, mercifully never adopted, assumed that U.S. government bonds are nonnegotiable.

To be sure, this is the way Mr. Trump has often operated in the private sector: borrowing to finance a high-risk venture, then aggressively seeking relief from creditors if his plans didn’t pan out, including via bankruptcy court. “I have borrowed knowing you can pay back with discounts,” he told CNBC. Playing that game with bankers, who, as Mr. Trump has repeatedly noted, are not “babies” but sophisticated profit-seekers, is one thing. Doing it with, or to, ordinary savers and investors the world over is quite another.

In what may be only the first of many political cleanup jobs for him, Mr. Trump’s new campaign finance chairman, hedge fund operator Steven Mnuchin, averred that “the government has to honor its debts.” The fact remains that Mr. Trump himself implied the contrary, setting a new record for economic recklessness by a major party presumptive nominee, in his first week in that role.