Reply #1: The problem only comes in two ways or at two times:

[forty6]

1) When no one will buy your debt, (bonds, T-bills, etc) We have never reached that point, but COULD reach it if the debt ceiling is NOT increased by Congress, ... at which time credit for EVERY AMERICAN, personal and corporate will become TWO TO THREE TIMES AS EXPENSIVE as it is now....(lighting off an inflation sparked recession of major proportions).

OR

2) When the government cannot collect enough money in receipts to pay for the debt service, (interest and principle payments on the bonds, etc.)as well as for the goods and services the government buys without using more debt.

The last time a Republican President sat on a budget surplus, (spent less than income), was Eisenhower.
The last time a Democrat President sat on a budget surplus was Clinton.

In BOTH of those times, we STILL BORROWED MONEY to run the government, buy stuff, pay elderly, etc. We just didn't spend everything we collected in taxes and borrowed, so the amount of borrowing went DOWN by a few billion (200 or so under Clinton, don't remember Eisenhower's surplus, but back then you could buy a loaf of bread for a quarter.

Which brings me to my LAST point, inflation has a way of making the national debt look smaller and smaller... and inflation will be the ONLY way we ever master the debt to reasonable levels. But society is MORE stable with LOW inflation, so it's a conundrum...more debt and less inflation, less debt and more inflation one or the other.