HONG KONG — An American labor union is pushing the United States to impose broad, steep tariffs on aluminum imports using a little-used but wide-ranging trade law that has riled the country’s trading partners in the past.

The effort by the United Steelworkers union comes with trade increasingly an election-year issue in the United States and elsewhere. More than three-quarters of the United States aluminum smelting industry that existed five years ago will have been idled or shut down by this summer as imports have surged, according to the union’s legal petition…