I managed to find one of Sander's position statements on this (but not Warren's so I'll accept an answer on that):
Donald Trump’s haphazard and reckless plan to impose tariffs on Canada and the European Union is an absolute disaster that will cause unnecessary economic pain to farmers, manufacturers and consumers in Vermont and throughout the country.
I strongly support imposing stiff penalties on countries like China, Russia, South Korea and Vietnam to prevent them from illegally dumping steel and aluminum into the U.S. and throughout the world. American steel and aluminum workers need our help, and they need it now, but not at the expense of farmers, workers, small businesses and consumers in Vermont and throughout this country. Given Vermont’s proximity to Canada and our strong trade relations with Canada, these policies will be especially harmful to Vermont and other northern border states. It simply makes no sense to start a trade war with Canada, the European Union and others who are engaged in fair trade, are not cheating and where workers are paid a living wage with good benefits.
So Sanders seems to have supported the measure on some countries but not on others.