The deficit is certainly a problem, and it is not sustainable. However, interest rates don't really have much of an effect on the deficit. Sure, the government has to borrow money at a rate determined by the market -- why would anyone invest in a government bond paying, say, 2% when you could get a AAA corporate paying a much higher rate? But Congress has unfortunately shown a willingness to borrow money regardless of the interest costs. Keep in mind that short-term government interest rates were in double digits for most of the Reagan 80s, when the deficit was massively expanding.