Percentage of Treasury Obligations Outstanding By Maturity
The Federal Government has $1.67T worth of debt that needs to be rolled annually, which creates considerable refinancing risk for the US government.
The reason that most argue that the US government can never default on its debt is that the Fed has a printing press and therefore an unlimited supply of money to buy Treasury debt. As it currently stands (partially because of operation twist) the Fed owns 6% of short term maturity bills, but holds 16% of all maturities. If there were a European-like buyer's strike on US government bonds, the Fed's balance sheet could have to expand by another $1.6T+ to absorb the short term maturities.