My colleague Kris Mitchener reminded me that back in the 1840s there were state defaults on debt.  I had thought that that legal avenue had been closed to states.  I am no constitutional lawyer, and of course with all things legal, there is never a clear answer.  But I found this short article helpful.  (The main issue in Puerto Rico as I understand it is not really about the government itself defaulting on general obligation bonds, but rather other government entities or instrumentalities defaulting.)

“There are two reasons why state governments currently cannot use the federal bankruptcy system to reorganize their debt. First, the federal bankruptcy code does not allow—and has never allowed—state governments to declare bankruptcy. Since 1937, the bankruptcy code has allowed ‘municipalities’ to declare bankruptcy. The term ‘municipality’ is defined in the bankruptcy code as a ‘political subdivision or public agency or instrumentality of a state.’ This definition is broad enough to include cities, counties, townships, school districts and public improvement districts. It also includes revenue-producing bodies that provide services which are paid for by users rather than by general taxes, such as bridge authorities, highway authorities and gas authorities. But it does not include state governments.“

The second reason stems from the U.S. Constitution. The contracts clause of the U.S. Constitution prohibits state governments from ‘impairing the obligation of contracts.’ As originally understood and enforced, this clause prohibited state legislatures from passing any laws to relieve either private debt or the state government’s own debt. Beginning in 1934, however, the Supreme Court began to interpret the contracts clause more flexibly and not as an absolute bar to state debt relief laws. Even under the flexible modern approach, however, the Supreme Court in 1977 reiterated that ‘a state cannot refuse to meet its legitimate financial obligations simply because it would prefer to spend the money (on something else.)’ Thus, were Congress to amend the federal bankruptcy code to authorize states to repudiate debt, the Supreme Court would then need to decide the novel constitutional question of whether such debt repudiation would nonetheless violate the contracts clause of Article I, Section 10.”