During the Nafta debate, all the major polls found +/- 70% of the public opposed. As the Congressional vote got nearer (and news coverage got more intensive and propagandistic) the number started dropping and reached its lowest level right before the vote. But I'm almost *positive* that it never got to 50% -- despite the fact that it was nearly impossible to find an "expert" type on TV making the anti-Nafta case.
Seth