Sure it is. By constitutional law, the debt interest must be paid first before any other obligations. The government takes in 8 to 10 times the amount of that interest, so they have plenty of money to sevice the interest. (Note I said interest, the principal owed is impossibly huge.) Obama is always talking about default, but he means "it's de fault of Bush". He can't allow actual default or he can be impeached.
I think the Constitution is being misread here. It applies to debt during the civil war. That US debt would be honored and not the Confederate debt
If you really think defaulting national debt is impeachable, why is piling on trillion of debt NOT impeachable? Because one of these days, debt interest will be over 100% revenue and servicing debt is impossible. How could default not be an option?