"main threat to US factory workers' jobs."

There is no such thing as threat - a factory worker who cannot compete MUST be selected out, and replaced by something that is cheaper. This is for the betterment of all, at the expense of the said factory worker. This is called reality and those who choose to prevent efficiency is only retarding progress.

Now, of course, the said factory worker isn't obsolete - they should "skill up" and do something that currently isn't achievable via low paid labour, or machines. This is where value is derived, and should be encouraged (ala, re-education).