I don't get the 14th Amendment argument

[FreeJoe]

How does it help? OK, so our existing debts are valid and have to be paid.

I don't know the real numbers, but lets just say that in August the Treasury expects to take in $200 billion, expects to pay $40 billion to cover our debts and expects to pay an additional $200 billion for stuff like government agencies, military expenses, etc. If you invoke the 14th admendment, you agree that the $40 billion in debt payments has to be made. That leaves you with $160 billion for everything else. How does the 14th amendment give the President or anyone else the authority to borrow more money to cover the difference?