Section 4 of the 14th Amendment says “The validity of the public debt of the United States, authorized by law, including debts incurred for payment of pensions and bounties for services in suppressing insurrection or rebellion, shall not be questioned.”
“Questioned” is pretty vague but there is certainly an argument (more here) to be made that the president could constitutionally disregard the debt limit. But disregarding the limit is only one way not to question the validity of the debt. Alternatively, one could argue that the way to keep with the constitution and the debt limit law is to cut spending or raise taxes.
…a modest approach for the President to take would be not to conclude that the debt limit is facially unconstitutional, but rather that it would be unconstitutional as applied, to the extent that it would prevent payment of interest on the debt. If the President took that position, the Administration would in effect continue to raise the debt limit as necessary to make payments on the debt. But when other bills came due, if there were insufficient funds to pay them, that would not justify the issuance of additional debt. The consequences of such nonpayment are sufficiently severe that the President and Congress could continue to play their game of chicken, but the worst case scenario would be a government shutdown, rather than a default. Perhaps the President can accomplish this even without invoking the Public Debt Clause, simply by prioritizing payments on the debt over other payments that come due, but his ability to do that depends in part on the timing of revenues and expenditures, and an announcement that the President will make payments on the debt despite the debt limit statute no matter what would calm the markets.
Have a Happy July 4.