The primary cause of most of the federal government's debt is deficit financing. Yes, it means that the root cause is "spending", but the debt has largely grown itself.
Any following figures noted as $B are rounded to the nearest $1B.
From '61-62 to '07-08 (the years for which I have figures from the government's own web site), there were only 14 years in which the operating balance (revenue - expenses) was a deficit, totalling $67B. The cost of public debt charges was $1,037B (yes, just over a trillion). (In '61-62 the public debt charge was $832M on an accumulated deficit - debt - of $14,825M.) I don't know what the pattern of deficit spending was prior to '61-62.
The reports show a peak debt of $562B in '96-'97. That was the year of a notable absolute cut in expenditures, following a string of remarkably restrained spending growth: from '92-'93 to '97-98 the program expenses were $122B, $122B, $123B, $120B, $111B, $114B. '97-98 was the year the net budget flipped from deficit to surplus for its decade-long run of surpluses.
When the debt was at its peak, the public debt charge for that year was $47B. (The debt charge actually peaked at $49B the year before, but interest rates were falling.) The debt charges as a percentage of debt were under 9%. That ratio peaked at 14% in '81-82 and was floating in the neighbourhood of 7% from '02-03 onward.
Some educated guesses:
1) By the time the current run of deficit financing is done in 3 years or so, we'll have a net debt of about $600B. At 7%, the cost of debt charges will $42 per year.
2) At early '80s rates, the debt charge cost could be over $80B.
3) Annual public revenues and program expenditures based on '07-08 should be in the neighbourhoods of $250B and $200B, respectively.
The main conclusions should be obvious. Much of the public revenue isn't paying for programs, and since interest rates likely have nowhere to go but up, that squeeze will become worse if not unsustainable.
My own take: I see no reason to expect remarkable revenue growth in the current global economic climate, so we won't easily improve the revenue side of the equation. We should be cutting public spending right now in large amounts, but we are not. I therefore expect a period of moderate to high inflation to commence in the next few years. What that does to savings, pensions, and other fixed incomes will not be pleasant.