We did pay the National debt down... once:
The result was that Martin Van Buren, (Andrew) Jackson's successor became a one-term President, following the panic and very serious depression of 1837. A depression caused, in great part, by a shrinking of private demand caused by Government austerity, the successful pay back of the debt, and the continuing policy of balancing the budget. In addition, every sustained attempt in American history to run budget surpluses was followed by either a depression or a recession. A summary of the depressing record is here.
To tell the truth though, neither the size of the national debt, nor the debt-to-GDP ratio are relevant to the capacity of the Government to spend either in the present, or in the future, because their size has nothing to do with the Constitutional authority of the US Government to issue currency and create money, and also has nothing to do with the solvency of the United States Government.
If you'd like to read more:
Paul Ryan's Deficit Reduction Fairy Tales: Part One