Back in 2007, Paul Krugman called him on it in a column entitled
"Played for a Sucker."
Lately, Barack Obama has been saying that major action is needed to avert what he keeps calling a “crisis” in Social Security — most recently in an interview with The National Journal. Progressives who fought hard and successfully against the Bush administration’s attempt to panic America into privatizing the New Deal’s crown jewel are outraged, and rightly so.
...
Which brings us back to Mr. Obama. Why would he, in effect, play along with this new round of scare-mongering and devalue one of the great progressive victories of the Bush years?
...
Mr. Obama wanted a way to distinguish himself from Hillary Clinton — and for Mr. Obama, who has said that the reason “we can’t tackle the big problems that demand solutions” is that “politics has become so bitter and partisan,” joining in the attack on Senator Clinton’s Social Security position must have seemed like a golden opportunity to sound forceful yet bipartisan.
But Social Security isn’t a big problem that demands a solution; it’s a small problem, way down the list of major issues facing America, that has nonetheless become an obsession of Beltway insiders. And on Social Security, as on many other issues, what Washington means by bipartisanship is mainly that everyone should come together to give conservatives what they want.
The last line of the column, although it's slightly off topic, deserves repeating:
"what Washington means by bipartisanship is mainly that everyone should come together to give conservatives what they want."