Tariffs have been falling since the WTO replaced GATT. Can anyone say Bill Clinton?
It's both parties. Nixon was the one who normalized trade with China. Neither party has an economic plan. Both parties are afraid to point to international trade as the problem. In some cases it may be apathy caused by putting globalization interests above American interests. But I suspect a lot of it, is simply fear of being labeled a protectionist. And fear of going against the "free trade" ideologues.