> The Framers of the Constitution added this [no Law impairing the Obligation of Contracts] clause in response to the fear that states would continue a practice that had been widespread under the Articles of Confederation—that of granting "private relief." Legislatures would pass bills relieving particular persons (predictably, influential persons) of their obligation to pay their debts.

However it seems that in the Great Depression the Supreme Court held it to be okay for States to modify morgages under the "emergency exception doctrine" http://en.wikipedia.org/wiki/Home_Building_%26_Loan_Associat...