Have you read the opinion section of any newspaper in the last three years? Yes? Then there is a better-than-even chance you have come across some impressive-sounding analyst predict that the United States is "turning into Greece."
Maybe it's been a while, so we'll recap. The short version of this story is that we'll spend ourselves into bankruptcy. The longer version says that too much public debt makes markets nervous. Nervous markets demand higher interest rates. Higher rates mean higher deficits and lower growth, both of which mean more burdensome debt. More burdensome debt makes markets even more nervous. And around and around we go in a vicious circle into insolvency.