William McKinley was a member of the Republican Party. During the late nineteenth century, Republicans strongly supported tariffs to protect growing industries within the United States from foreign competition. The McKinley Tariff was passed into law in 1890, and it dramatically increased the tax rate on foreign products. While many business owners supported this legislation, American consumers generally opposed it, as prices increased for goods. During the late 1800s and early 1900s, the Republican Party and the Democratic Party continuously battled over tariffs. American opposition to the McKinley Tariff was so high that President Benjamin Harrison, a Republican, may have lost reelection in 1892 partly because of his support for the tax.
via Tax History Lesson: The McKinley Tariff | Tax Foundation
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