Free trade is, in principle, a good thing. Basic economics, that.
But! Not all countries play by the same rules. Free trade with a country that has slave labor? Not cool, and not fair to your own labor-intensive industries.
And, you can nobble your own businesses. Environmental protection and workplace safety are important; free trade with a country which has no standards (or which fails to enforce them) gives that country an advantage, until things get so bad there that the peasants revolt. And, having decided that these things are important, making compliance any more expensive than necessary makes your own businesses uncompetitive, and provides a powerful motive for offshoring, harming your own workers*.
So, sure, let's implement the corporatist Democrats' dream of "a hemispheric common market, with open trade and open borders" while imposing a $15/hour minimum wage and vast regulatory burdens on domestic businesses great and small. We'll see soon enough how much that benefits the working classes.
* Offshoring ain't so good for the Company, neither; employing a factory full of furriners with their own ideas about language, work hours, holidays, and so on opens a whole new can of troubles. But maybe it's less trouble than trying to run a factory in Ohio, never mind California.