Democrats betray their labor ties because globalists have bought them off, therefore if you support free trade you have been bought off, too.
Global taxation would be a top down system, while free trade is nothing more than capitalism applied internationally. In other words, free trade is not something imposed by a world government, but the opposite. It means merely that businesses and individuals are allowed free use of their own capital.
That being said, the whole question of free trade is kind of academic, since it can only exist between free countries with free markets, and there are not very many of those.
Additionally, IMO, the question of national security has to be looked at - farming out all our heavy industry to foreign countries makes us vulnerable.
However, I do believe that free trade is generally beneficial for the same reasons that the free market is beneficial.