Doubtlessly, the question will emerge as a campaign issue. But any intellectually honest answer - perhaps futile in today's politically charged climate - will admit that no single cause explains the change. We now have evaluations from the CBO and two nonpartisan groups: the Committee for a Responsible Federal Budget (CRFB) and the Pew Fiscal Analysis Initiative. They all point in the same direction.
For starters, a weak economy was the largest cause. The CBO attributes $3.2 trillion of the $11.7 trillion shift (about 27 percent) to "economic and technical changes." "We overestimated how good the economy would be, even before the Great Recession," says Marc Goldwein of the CRFB.
Together, the weaker economy and 2001-2003 tax cuts explain 40 percent of the debt shift. Here's how Pew allocates the rest. Its estimates parallel the CBO's and the CRFB's, which either cover slightly different time periods or use slightly different budget categories.
Iraq and Afghanistan wars: 10 percent
Increases in discretionary domestic spending: 10 percent
Other increases in defense spending: 5 percent
Obama stimulus: 6 percent
2010 tax cuts: 3 percent
Medicare drug benefit: 2 percent
Other tax cuts and means of financing: 12 percent
Higher interest costs on larger federal debt: 11 percent
So, most theories (often partisan) of the $11.7 trillion shift turn out to be wrong, exaggerated or misleading. There were lots of causes; no single cause dominates.
One other thing: Even projecting surpluses from 2002 to 2011, the CBO cautioned then that large deficits would ultimately return.
"Over the longer term," then-deputy CBO director Barry Anderson testified before the Senate Budget Committee in January 2001, "budgetary pressures linked to the aging and retirement of the baby boom generation threaten to produce record deficits and unsustainable levels of federal debt."
Unfortunately, that hasn't changed.