The interest debt is represented on the far left of the red bar. The Green bar represents revenues taken in each month. There is no worry about defaulting. It's just a scare tactic that the balanced media has perpetrated to present this debate in a very unbiased and balanced scare tactic way:
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"You and I are told we must choose between a left or right, but I suggest there is no such thing as a left or right. There is only an up or down. Up to man's age-old dream – the maximum of individual freedom consistent with order – or down to the ant heap of totalitarianism." -Ronald Reagan