It is notoriously difficult to predict the specific onset of a sovereign debt crisis. A quiescent scenario now--probably based on expectations that a crisis will be averted--does not mean that the actual crisis that would result from a failure to raise the debt ceiling would be insubstantial. Any disruption that is the equivalent of around 10% of GDP is bound to have a large macroeconomic impact. Any rise in yields could significantly widen the nation's long-term imbalances. Psychology can amplify the turmoil, hence a $4 trillion deficit reduction plan might be credible at present, but if a crisis were underway, such a plan might no longer be perceived as credible, especially in the face of higher interest rates and the political dysfunction that led to the self-inflicted crisis.