Why did some americans oppose nafta?

The reason SOME people think it's bad for America is because they take a narrow view at the bill. Yes, it's true some people were temporarily unemployerd due to outsourcing jobs overseas. But EVERYONE, including those people, now pay much, much less for goods that otherwise would've been more expensive. And with that extra money, they can spend more, save more, or generally have more discretionary spending-- which keeps the economy chugging along. That benefit of free trade far, far, far outweighs the job losses. And even those job loses are temporary. 

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