Brad DeLong, in Violating the Constitution, points out that Section Four of the Fourteenth Amendement of the Constitution of the United States begins,
Section. 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.
Shall not be questioned. Remember that the next time you hear a right-winger talking about the bonds that make up the Social Security Trust Fund being somehow different from other government bonds.