If they can't sell them? Did you hear about how today's auction was 3 times oversubscribed? I don't think the Treasury is going to have any problem selling the new debt.
Right now it is easy for them to sell Treasuries because of the flight to safety and the strong dollar. However remember they need to sell about 3x as many next year as they have this year so far. That's a lot. I expect the dollar to weaken at some point and inflation to emerge. This will cause a flight out of Treasuries back into hard assets- commodities and PMs. Then they will either have to force Americans to purchase Treasuries or openly monetize the new government debt. Or both, as doing the latter will also destroy the market for Treasuries.