A sudden fiscal crisis

"Growing debt also would increase the probability of a sudden fiscal crisis, during which investors would lose confidence in the government's ability to manage its budget and the government would thereby lose its ability to borrow at affordable rates. Such a crisis would confront policymakers with extremely difficult choices. To restore investors' confidence, policymakers would probably need to enact spending cuts or tax increases more drastic and painful than those that would have been necessary had the adjustments come sooner."

This didn't come from some alarmist website it came from the CBO.