But there is something interesting- not only have conservatives moved to the other side of the issue, but so have progressives. Now it is the progressives and the unions they control who tend to favor protectionism. I have a theory as to why this switch occurred.
Why both sides have changed probably has a lot to do with the evolution of modern business conditions; the progressives have always been anti-capitalism and the conservatives pro-business. Back in the late 1800s and early to mid 1900s protectionism was pro-business (and as such opposed by those who wanted to destroy capitalism). Now, in the world made small due to technology, it is protectionism which can cripple business and has been adopted by the progressives.
I think that this fits, particularly when considering the case of Buchanan, staunch protectionist, who has said that unfettered capitalism is a fiercely destructive force.