Here’s a Treasury Dept. page on Frequently Asked Questions (FAQ) about the Debt. I haven’t checked through all the pdf’s there, but I’m pretty sure that many of the instruments of debt are for as long as 20 or 25 years.
And increasing the debt by issuing bonds has the equivalent effect to printing money. With more money in circulation more goods can be bought, more factories opened, more people paid etc. It’s somewhat safer than printing money in that it has less inflationary pressure - the debt will be withdrawn after a certain number of years rather than permanently increasing the money supply.
The notion goes back to John Maynard Keynes during the Depression years. His theory essentially says that the government should spend money during business slumps to maintain equilibrium and then reduce spending when the economy recovers.
If spending is not reduced, the consequences according to theory would be that the interest on the debt would itself become a drag on the economy.
This all appears to work, but less well in the real world than in theory. It took the incredible amount of spending during World War II to break out of the Depression, for example, and that was on the part of both business and the government. As for what happened in the 1980’s, well, that’s real GD territory.
However, it is safe to say that the major influence in the 1980’s was the Chicago School of economists which led to Milton Friedman and his monetarist policies. They believe that the market is a better governor of economic policy than the government.
Of course, I’m so not a real economist that I don’t even believe that the stock market is rational. 