First, it does not simply say that the national debt must be paid; it says that its "validity ... shall not be questioned." Only one other section of the Constitution--the Thirteenth Amendment's proclamation that "either slavery nor involuntary servitude ... shall exist within the United States, or any place subject to their jurisdiction"--is as unqualified and sweeping.
Second, it suggests a broad definition of the national debt: "...including debts incurred for payment of pensions and bounties for services in suppressing insurrection or rebellion."
From this language, it's not hard to argue that the Constitution places both payments on the debt and payments owed to groups like Social Security recipients--pensioners, that is--above the vagaries of Congressional politics. ...
If Congress won't pay them, then the executive must.http://www.theatlanticwire.com/politics/2011/06/debt-ceiling-unconstitutional/39408/