1. The Second Liberty Bond Act of 1917 that establishes the debt ceiling;
2. The Federal Reserve Act that prohibits the Fed from lending directly to the Treasury; or,
3. The 14th Amendment of the Constitution that holds that the debt of the United States government, lawfully issued, will not be questioned.
They have to break a law. At the end of the day, officials will avoid violating the Constitution by indicating that they have been given inconsistent instructions and are obeying the one with the most important precedent.
Basically in Reinhart's formulation, Lew will opt to break the debt ceiling law, citing constitutional obligations to continue servicing the debt.
And then of course we enter into level never-neverland with respect to the budget.
An interesting analysis. Constitutionally, the debt must be serviced, so any law preventing that from happening is unconstitutional. Never-neverland is a good name for a world like that.