Yeah, it was the national debt Romney said Obama had raised. Do you believe this because Romney said it? He said quite a number of false things.
Politifact gives this accusation a "mostly false" rating:
http://www.politifact.com/new-jersey...evious-43-pre/
CBS News says "the Debt rose $4.899 trillion during the two terms of the Bush presidency. It has now gone up $4.939 trillion since President Obama took office." (
http://www.cbsnews.com/8301-503544_1...an-under-bush/) Not exactly the same numbers, but that doesn't sound like "more than all 43 previous presidents combined."