Article 14, part 4: The validity of the public debt of the United States, authorized by law, including debts incurred for payment of pensions and bounties for

Article 14, part 4: 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. To be fair, there has been some debate into how this is interpreted as far as the constitutionality of debt ceilings if they would cause the government to default on its debt. The US has always paid its debts, so the question hasn't been litigated and decided in court. In any case, constitution or not, the biggest reason not to default on debts is that doing so makes it far more expensive to borrow in the future.