It is that progressive Dems some time ago glommed onto the idea that protectionism is "progressive." It has been going on so long and has become so ingrained that Bernie Sanders has been running around bragging about how he is more protectionist than Trump. Elizabeth Warren has been a bit more subtle about it, calling to renegotiate all existing US trade agreements to make them super strong on labor and environmental standards.
The problem is that one of the biggest disasters of the Trump presidency has been his trade wars, now pushed further with his latest move to raise tariffs on another $300 billion in Chinese imports. Stock markets and oil markets took huge dives all over the world on this. The Fed has just cut interest rates to offset the negative effect on the world economy of Trump's trade wars. Trump has delivered a big fat zero in terms of anything positive from his protectionist moves, and even industries that were crying for protection, such as steel and and autos, are now complaining about his trade wars. And this has done a big fat zero for workers as well, whom supposedly our great "progressive protectionists" claim they are spouting their now completely irrelevant drivel.
This is going to be one of the biggest issues in the coming campaign, and while so far almost nobody is focusing on it, both Sanders and Warren are complete and totally worthless disasters on it. I find this very frustrating given that on so many other issues they make a lot of sense.