Robert Reich has a good post on NAFTA and the dilemmas of trade policy more generally. His main argument is a bit hard to summarize and not very long, so I'd suggest you read it yourself. I'll just quote the newsworthy bit where he offers his recollection of where Hillary Clinton stood on NAFTA during her husband's presidency:
The answer is HRC didn't want the Administration to move forward with NAFTA, but not because she was opposed to NAFTA as a policy. She opposed NAFTA because of its timing. She wanted her health-care plan to be voted on first. She feared that the fight over NAFTA would use up so much of the White House's political capital that there wouldn't be enough left when it came to pushing for health care. In retrospect, she was probably right.
That seems reasonable enough. Another way of looking at it that's less dependent on the precise issues of timing is simply that the Clinton administration got scandalously little from the business community in exchange for supporting things like NAFTA. Whatever the order of operations, you would hope that in the future if a Democratic administration winds up bucking its labor allies to pass a business priority on trade policy, that some kind of guarantees are forthcoming that groups that benefit from the deal will deliver some moderate Republicans to cast tough budget votes or to participate constructively in health care legislation.