Removing tariffs on manufactured goods between the US and China, for instance, strongly benefits people who can set up factories to take advantage of the cheaper labor and materials. It weakly benefits consumers who get slightly (if at all - corporations might just take the difference as profit depending on the level of competition) cheaper goods, and it disadvantage all of the working class that now has to compete with cheaper Chinese labor.
This can be mitigated by income redistribution, funding for job skills retraining, and so on (although it kinda sucks to find yourself back at the bottom rung of a new field at 40 because you can't find job opportunities anymore) but, historically, these programs haven't received any funding, so it's pretty safe to assume they won't exist until proven otherwise.
