If people don’t buy foreign cars because they are too expensive, then no one. For the union there’s no intrinsic downside to such a protectionist action.
It is never possible that the producers and their labor will benefit more than the aggregate harm imposed on consumers by a tariff.
The cost of a tariff on the economy will always be more expensive than the benefit it gifts to a select few. However, the political reality of the cost spread out to many people compared to the larger per person benefit enjoyed by a smaller number of cronies means that tariffs will almost always be popular.
Looking at vehicle production through the single dimension of economics without considering geopolitics seems naive.
Moreover, longer term tariffs leads to domestic suppliers not being as efficient and hence fall behind international competition. Thus, domestic consumers get garbage cars and become grumpy at the people who supported the tariff in the first place.