The Constitution vests financial powers in Congress, not in the President. It's Section 8 of Article I.
Link to Section 8:
http://topics.law.cornell.edu/constitution/articlei#section8Link to 14th:
http://topics.law.cornell.edu/constitution/amendmentxivOnly Congress has the authority to borrow money on the credit of the United States. The authority of the President to do so lies solely in his function as the Executive to carry out the directions of Congress.
The section of the 14th people are citing just 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.
Questioning the "validity" of the public debt means questioning the lawfulness of it. But no one is questioning the validity of the public debt already extant. Failure to raise the debt limit does not functionally require default on the debt, so even that argument is completely bogus. Instead, failure to raise the debt limit just means that spending has to be cut.
The SC has constantly ruled that in areas in which the Constitution gives joint authority over a function, when Congress legislates that will generally trump the authority of the Executive to do differently. But in this case, the Constitution gives the Executive no authority whatsoever to borrow money. Further, under the necessary and proper clause, it is clear that Congress can set a current debt limit and only authorize the Executive to conduct borrowing operations up to it.
Therefore criticism of Obama on this score is completely wrongheaded.
Text of Article I, Section 8:
The Congress shall have power to lay and collect taxes, duties, imposts and excises, to pay the debts and provide for the common defense and general welfare of the United States; but all duties, imposts and excises shall be uniform throughout the United States;
To borrow money on the credit of the United States;
To regulate commerce with foreign nations, and among the several states, and with the Indian tribes;
To establish a uniform rule of naturalization, and uniform laws on the subject of bankruptcies throughout the United States;
To coin money, regulate the value thereof, and of foreign coin, and fix the standard of weights and measures;
To provide for the punishment of counterfeiting the securities and current coin of the United States;
To establish post offices and post roads;
To promote the progress of science and useful arts, by securing for limited times to authors and inventors the exclusive right to their respective writings and discoveries;
To constitute tribunals inferior to the Supreme Court;
To define and punish piracies and felonies committed on the high seas, and offenses against the law of nations;
To declare war, grant letters of marque and reprisal, and make rules concerning captures on land and water;
To raise and support armies, but no appropriation of money to that use shall be for a longer term than two years;
To provide and maintain a navy;
To make rules for the government and regulation of the land and naval forces;
To provide for calling forth the militia to execute the laws of the union, suppress insurrections and repel invasions;
To provide for organizing, arming, and disciplining, the militia, and for governing such part of them as may be employed in the service of the United States, reserving to the states respectively, the appointment of the officers, and the authority of training the militia according to the discipline prescribed by Congress;
To exercise exclusive legislation in all cases whatsoever, over such District (not exceeding ten miles square) as may, by cession of particular states, and the acceptance of Congress, become the seat of the government of the United States, and to exercise like authority over all places purchased by the consent of the legislature of the state in which the same shall be, for the erection of forts, magazines, arsenals, dockyards, and other needful buildings;--And
To make all laws which shall be necessary and proper for carrying into execution the foregoing powers, and all other powers vested by this Constitution in the government of the United States, or in any department or officer thereof.