Democrats are urging the president to "invoke the 14th Amendment" to the US Constitution. The intent here is to give the President the authority to unilaterally raise the debt ceiling. They use the Section 4 which 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. 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.
Apparently, they miss that whole "authorized by law" part. If Congress doesn't authorize it, it's not "authorized by law".