You do realize that until Obama got elected, many leading Democrats vocally supported protectionism/higher tariffs, especially on China, right? Even under Obama, some Democrats, such as Chuck Schumer, openly called for a strongly protectionist trade policy toward China.
If tariffs are so bad, how did the U.S. become the world's leading economic powerhouse during all those decades when we had an ardently high-tariff trade policy? How'd that happen? How have and other nations prospered while following a high-tariff trade policy?
But now, reflexively, robotically, most Democrats whine about Trump's America-first high-tariff trade policy just because he's the one doing it.