14th Amendment

About our national debt, the Constitution says this in the 14th amendment:

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. But neither the United States nor any State shall assume or pay any debt or obligation incurred in aid of insurrection or rebellion against the United States, or any claim for the loss or emancipation of any slave; but all such debts, obligations and claims shall be held illegal and void.

This is obviously a post-Civil-War amendment; it was adopted in 1868. The parts about insurrection and rebellion can be safely ignored. Which leaves us with this:

The validity of the public debt of the United States, authorized by law, shall not be questioned.