It isn’t entirely clear that the debt ceiling is constitutional.  Neither, however, is there a clear remedy for violating it.  Bartlett (and in an op-ed linked to by him, Tribe) discuss some alternatives which may be “more or less unconstitutional” than default — such as either the President or the Treasury choosing to prioritize payments.  A bit long but well worth reading:  <http://capitalgainsandgames.com/blog/stan-collender/2778/must-read-bruce-bartlett-debt-limit>

In particular, it comes down to how the the Fourteenth Amendment, section 4, gets interpreted:

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.

Of course, it’s up to the Supreme Court to interpret this.  And in the interim, for example, if prioritizing payments decisions need to get made in the emergency situation of an actual impending default, a separate Constitutional issue arises – *who* does the prioritizing and paying, since Article 1, Section 8 makes explicit that it’s Congress, not the President, that has the “power … to pay debts”.  If this means that, in conjunction with the 14th amendment, the debt limit is unconstitutional and that the government is obligated to continue to pay on its debts, how, exactly, does Congress get held to this and forced to pay those debts?  And if they don’t in a timely manner, is it unconstitutional for the Treasury (under the *executive* branch, i.e., under the President) to go ahead and make payments regardless?

Your guess is as good as mine.  We have only a matter of hours to find out what happens next.