Over at the Huffington Post, Sam Stein marshalls some evidence suggesting Hillary opposed NAFTA from the beginning. It's a compelling brief--particularly this quote from Mickey Kantor (which also caught Ben Smith's eye):
"In August in 92, we had to make a decision," Mickey Kantor the former U.S. Secretary of Commerce, Clinton adviser, and free trade advocate recalled for the Huffington Post. "President Clinton had to make a decision as governor, whether or not he would support [George H.W. Bush's] NAFTA, and of course he did... Hillary Clinton was one of the great skeptics in the discussion as to whether he should do. So she was always skeptical beginning in 1992 and onward."
Which raises an obvious question: If Hillary has been a longtime NAFTA skeptic, why hasn't she made much of this herself? The obvious answer is that she's running on the (generally solid) legacy of her husband's administration, and she can't claim some parts of that legacy while junking others.
I'd suggest two other reasons. First, talking up her opposition to NAFTA might suggest she wasn't very influential in the Clinton administration--it passed, after all--which would undercut a key argument about her experience. Second, it might also play into the narrative Obama has been trying to impose--which is that Washington is a place where people show up with ideals, only to have "boiled" away. The NAFTA episode could be spun as a story about Hillary's accommodation to "the system," which is what Obama vows to change.
I'd add that neither of those critiques would be completely fair. You can be influential without winning on ever major battle. And you can be a good soldier when you lose without selling out. But these would be reasonable ways to use Hillary's NAFTA-opposition against her, and so I'm not surprised she didn't want to broach it.