You need to read about the difference between internationalism and globalization, the left doesn't oppose a united worldwide community it opposes globalization. No fucking way in hell Marx would be in favor of free trade agreements. "Free trade" btw doesn't necessarily mean no tariffs, free trade at Marx's time meant a very different thing.
See here's part of that difference in tone thing that The Bore just doesn't do and now you make me have to go and write a post like this. That the respected posters will take issue with. Thankfully, I have no self-respect.
First of all, the Marx quotes were obviously half-joking because you said "actual leftist economist" and how can you not go straight to Karl for that?
Free trade at Marx's time actually was far more simple than we're making it and did mean no tariffs/prioritization like Imperial Preference and that was it. Marx, Engels and the US and UK governments
all argued that breaking the UK's protectionist and favoring policy for trade within the Empire would greatly benefit the United States' relative position, which is why it was deliberately broken during World War II.
I'm the last one to support the details of free trade agreements as being free trade since you know, free trade doesn't need thousands of pages of policy attached to it. The fact is there's really no leg to stand on in opposition to free trade (as understood by all those brainwashed corporate neocon economists) except for nationalist class-propping up ones like you're making when you keep throwing out the presumably American "middle class" as if they're some special class that should be immune from natural forces because they got their first or got their last or whatever.
If you're an actual leftist, not a liberal who's really a "right-winger" than you shouldn't be opposed to wealth distribution.
Since definitions and estimations aren't perfect I'll be choosing a single one, Pew estimates ~125 million are in the American "middle class." If we take $10,000 from 100 million Americans that means
5 billion of our other former humans on Earth could get a maximum $200 in a straight distribution.
That's why I asked about national borders. And your response, like above, is that what you mean by "protectionism" isn't "protectionism" as commonly understood but some other unknown thing that still halts trade across countries but allows for high living standards everywhere but only for the middle class within each nation. Something that's never occurred anywhere under protectionist systems.
To wrap back around to Marx, the reason he argued in favor of free trade is that it would break the
entrenched classes hiding behind
state protectionism. As those were considered synonymous with the nation-state, there could be no worker uprising as it would be quashed as acting against
the nation not the exploiting class. (Which it did!)
His problem was of course that he believed that voices that represented natural forces spoke to and through him, but he wasn't about to throw out
everything known about the benefits of free trade over protection even at that time when he formulated his materialist theory, you can't get to capitalism until you break away from mercantilism. And you can't get to socialism without capitalism.