This is a bit of an obvious point, but I want to make it anyway: pretty much every liberal, even those who generally support the idea of adopting chained CPI as a more accurate measure of inflation, should be opposed to President Obama's proposal to adopt chained CPI.
The reason is simple: chained CPI represents a cut in the growth rate of Social Security benefits. It's arguably something that's worth accepting as part of a larger bargain that would cut benefits a bit and raise taxes a bit in order to improve Social Security's finances, but it makes no sense on its own. Social Security is separate from the rest of the federal budget, and its benefits should never be horse-traded away for miscellaneous changes elsewhere.
If Republicans are ever in a mood to consider a serious Social Security deal that's designed to improve its solvency in a balanced way, that's fine. I'm ready to listen. But that's not on the table. Until it is, chained CPI shouldn't be on the table either.