Since the 1990s, which political parties have generally supported free trade and which have generally oppose it? Why?

Democrats have been opposing free trade in its compete sense while the republicans have been for it.

Explanation:

The American left is economically in the socialist front while it is liberal in its civic front and is broadly represented by the democrats however, none of these classifications hold much weight but it is the general trend.

The conservatives also tend to be more into the free trade and capitalism and form the republican side of American politics and the red states.

These people are the ones that believe in free trade because they believe that US is the land of  capitalism and it must flourish unaided and it w ill help the community of people.