After assuring Americans he is not running for president “to make things unstable for the country,” the presumptive Republican nominee, Donald J. Trump, suggested that he might reduce the national debt by persuading creditors to accept something less than full payment.

Asked on Thursday whether the United States needed to pay its debts in full, or whether he could negotiate a partial repayment, Mr. Trump told the cable network CNBC, “I would borrow, knowing that if the economy crashed, you could make a deal.”

He added, “And if the economy was good, it was good. So, therefore, you can’t lose.”

Such remarks by a major presidential candidate have no modern precedent. The United States government is able to borrow money at very low interest rates because Treasury securities are regarded as a safe investment, and any cracks in investor confidence have a long history of costing American taxpayers a lot of money.

Experts also described Mr. Trump’s proposal as fanciful, saying there was no reason to think America’s creditors would accept anything less than 100 cents on the dollar, regardless of Mr. Trump’s deal-making prowess.