I'm embarrassed that I haven't thought of/heard of this sooner, via Reason:
Republicans and some budget analysts noted that this highly touted goal is not particularly ambitious: This year's budget deficit [$1.75 trillion under Obama's fiscal plan] is bloated by spending on the stimulus package and various financial-sector bailouts, expenses unlikely to be repeated in future years. The nonpartisan Congressional Budget Office recently predicted that the deficit could be halved by 2013 merely by winding down the war in Iraq and allowing some of the tax cuts enacted during the Bush administration to expire in 2011, as Obama has proposed. That alone would cut the deficit to $715 billion, according to the CBO.
"It's easy to cut the deficit in half after you've quadrupled it," said Brian Riedl, a budget analyst at the conservative Heritage Foundation. "The end of the recession, the drawdown of Iraq spending and the end of temporary stimulus spending will by itself cut the deficit in half."