This is the doomsday scenario that is constantly thrown around in this forum. The answer, of course, is that as we all become poorer and poorer (as they'd have us believe) we'd no longer be able to afford imports.
And because the dollar would be so cheap, of course then the business would return here.
But, guess what? It's already happening and we're not even poor. The dollar is declining in value relative to other currencies and our exports are increasing. Manufacturing is up. When left alone, the system is self-regulating.
The other point the protectionists don't take into account is that if I'm forbidden by law to purchase labor overseas (outsourcing), I might just shut down my business altogether.
I could take my money and simply invest over there. I don't have to keep my money tied up in plants and a labor force that aren't profitable. Maybe we have to regulate that, too?