Help erase USA's national debt
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If you pay down the debt, the other problems won't go away because we are spending a lot more than we're receiving in tax revenue.
Honestly, I believe that the best dollar spent to get the debt under control is a dollar spent to get a new senator or congressman elected that will actually do something to change things in Washington.
anyway...
Obviously the debt looks unsustainable now and it does seem like it could never be paid off or even be maintained. But the US debt (relative to GDP) was higher after WW2 and it had a serious debt put in it during the postwar boom (due to a mixture of economic growth and negative real interest rates). So, never say never.