Obama particularly objects to language in the resolution saying that it's a "critical national interest of the United States" to keep Iran from creating a Hezbollah-like force in Iraq. Obama himself has said that it "is in our national interest to prevent" Iran from doing that. At bottom, Obama seems to believe that Congress shouldn't say things that he thinks are true, lest the Bush administration take comfort in it. How divisive, not to mention schizophrenic.
His other avenue of attack on Clinton is that she's vague about her plans to keep Social Security solvent. She supports a favorite Washington placeholder, a bipartisan commission. He's upfront about his support of increased payroll taxes. Hillary is dodgy about it because she's thinking about running in a general election, something Obama doesn't have to worry about overmuch.
Maybe he'll yet revive. No path to the nomination is ever as smooth as Hillary Clinton's looks to be right now. But Obama likely will go the way of that other earnest liberal, Bill Bradley, who high-mindedly let himself be run over by Al Gore in 2000. Oh, well. It'll be a great book.