Krugman gives us a graph:
What you see are the core drivers of Obama's spending – an understandable jump in money spent on recession-era safety nets. Much of this is automatic. The collapse in revenues and the debt explosion of the last couple of years have been almost entirely due to the recession which Obama inherited, and increasing costs from entitlements programs which Obama did not invent.
This is pretty obvious. Which is why, almost immediately upon his coming to office, propagandists like Glenn Reynolds used projected debt levels to indict Obama for being a leftist.