The fallout from President Donald Trump’s tariffs and China’s countertariffs—which formally went into effect on Friday—will have the greatest impact on the U.S. counties that voted Mr. Trump into office.

The U.S. tariffs on China will initially hit about $34 billion of goods, with plans in place to raise that total to $50 billion. The tariffs will fall mostly on Chinese aerospace products, information technology, auto parts and medical instruments. Beijing is retaliating with tariffs on $34 billion of American goods, aimed...