FactCheck.org:  Dubious Denver Debate Declarations:
Obama and Romney Swap Exaggerations and False Claims in Their First Meeting:

• Obama accused Romney of proposing a $5 trillion tax cut. Not true. Romney proposes to offset his rate cuts and promises he won’t add to the deficit.
• Romney again promised to “not reduce the taxes paid by high-income Americans” and also to “lower taxes on middle-income families,” but didn’t say how he could possibly accomplish that without also increasing the deficit.
• Obama again said he’d raise taxes on upper-income persons only to the “rates that we had when Bill Clinton was president.” Actually, many high-income persons would pay more than they did then, because of new taxes in Obama’s health care law.