... I would submit that it's fine to continue borrowing indefinitely, and even to borrow more and more each year, provided that the growth in the debt doesn't get too far ahead of the growth in GDP. ...
I would agree with this, but want to note that there is potentially a strong and disasterous link between GDP and the increased borrowing needs: I.e. The interest rate required to persuade foreigners to continue financing the debt. They are now showing (especially their government's central banks) a recognition that buying Treasury PAPER is a way to LOSE purchasing power. For details on this, see my thread Sovern Funds..." at:
http://www.sciforums.com/showpost.php?p=1518666&postcount=1
I.e. interest rates must rise to get foreigners to buy Treasury PAPER, instead of REAL ASSETS. This is the mechanism of the "disasterous link."
As interest rate increases, the cost of new borrowing goes up linerally with the rate and of course the value of the old debt's principle drops, causing the holders to LOSE purchasing power even more rapidly, and want even higher rates to compensate when they "roll" their old US bonds into new bonds, if they are even willing to roll, instead of collect. If they decide to collect, then the US Treasury must sell even more new bonds to pay them, requiring still higher interest rates etc. This alone is a dangeous postive feed back system, but that is only one side of the "disasterous link" coin.
As the interest rates go up, the GDP goes down as business slows, people can not buy their "starter home" or "move up" etc. This also adds to the dramatic effect on the Debt/GDP ratio.
To make a simple analogy, the US has had a great time skating along while others financed their rental of the skates and few recognize that the US is already on thin ice. I can see (and have documented in several threads here) that the ice is already cracking not far from the skater. I.e.
Rapid growth of Sovern Funds as central banks get out of US Treasury PAPER, Dropping real wages, negative saving rate, factories closing, out sourcing of jobs, 12 year low in housing starts, historically high inventory of unsold homes, house prices dropping in every city in inflation adjusted dollars, London now leading financial service center, not NYC, Auto sales by Detroit very slugglish and slowing, Baby Boomers retiring and claiming their Social Security instead of paying taxes as in their peak earning years. There are many other "cracks" showing to add if I took longer to think about it. (by edit - an unending war with growing expenses)
Where would the US debt to GDP ratio be if interest rate doubles (to not even near the historic highs) and GDP drops correspondingly?
It appears to me that US is just now entering into a very disasterous positive feed back system and can do nothing about it as foreigners learn that the dollar is not a sound investment nor a safe store of wealth.
Note also I did not mention above that China has already enough reserves to destroy the dollar if it chose to, but if done now, that would be painful to China also. They must wait until their rapidly growing middle class (both in numbers as ~ a million pesants move to the cities each month and as their individual salaries rise) can buy all the output of China's factories. If they learn what credit cards are and use them their purchasing power will exceed that of ALL Americans. China will not need the US market and not continue to accept PAPER for real goods. - This will also raise the cost of living for Joe American and he will not be able to buy as many US made products. - more slowing of GDP.
I do not like any of this I see coming, but logic forces that conclusion, which is a dramatic break with the past.