Quote from Ricter:
In such a scenario competition for dollars is elevated, and that's likely because the economy is booming, ie. inflating. So revenues will be up and social safety net spending will be down--less borrowing will be required. (I think stagflation could return.)
As for the $20 trillion in debt, which cannot come due all at once anyway, the US has many, many times that much value on the asset side of the books. Worst case scenario, where spending could not be sufficiently cut (and printing is impractical), assets could be sold. Not ideal, but also not likely.