Wait a second... so they did support it. I understand the circumstances. Circumstances change.
I should point out that the slave owners were the ones that were primarily in favor of free trade. Their agrarian economy forced them to import most of their goods, and the north generally was protectionist. But that opens up a states' right debate that I don't want to deal with now.
All of which has nothing to do with the current discussion...
So what changed between 1940 and 1960 that suddenly ushered in the free trade era? Why did we go from the world's biggest exporter to the world's biggest importer?