As I know it, protectionism (protecting domestic markets from foreign trade) is a form of isolationism, that is the opposite of imperialism. Why did McKinley pass McKinley's tariff if he advocated for imperialism? Thanks.
First, yes, the McKinley Tariff was definitely protectionist. Although from the midwestern state of Ohio, McKinley was firmly on the side of the Gold Standard and eastern banking interests, and saw protection of American industries as more important than avoiding price increases for consumers. That tariff increase would be the reason the Democrats and Grover Cleveland would come to power in the following election.
However, that was Congressman McKinley in 1890. The Spanish-American War came in 1898 ,years later, and came to President McKinley. When it did, he played the part of a very reluctant imperialist:
First, in the cause of humanity and to put an end to the barbarities, bloodshed, starvation, and horrible miseries now existing there, and which the parties to the conflict are either unable or unwilling to stop or mitigate. It is no answer to say this is all in another country, belonging to another nation, and is therefore none of our business. It is specially our duty, for it is right at our door.
Second, we owe it to our citizens in Cuba to afford them that protection and indemnity for life and property which no government there can or will afford, and to that end to terminate the conditions that deprive them of legal protection.
Third, the right to intervene may be justified by the very serious injury to the commerce, trade, and business of our people, and by the wanton destruction of property and devastation of the island.
McKinley stated position was that the Cuban revolt against Spain had endangered US citizens and US property in Cuba: that Spain had not allowed a peaceful settlement, and that the US, while not acting to take actual possession of the island, did not think the Cubans capable of having their own government. He would use similar rather racist language when the US took control of the Philippines: they should be free from tyranny, but they're not able to have their own government yet-so we'll step in and provide them with one.
This paternal language had been deployed by other Western countries to cover imperialist actions, and the war was quite convenient to the economic interests of the US: Cuba was right next door, the Philippines provided a valuable coaling station for US ships. While a tariff was to be a protection for home industries, those industries needed an outlet for their products- if not colonies, something like them: a prime motivation for imperialism. Neither Cuba or the Philippines had any competitive industries, but they did have customers. McKinley simply voiced the position that he was very reluctant, but America clearly had no choice. But it was rather transparent.... As Peter Finley Dunne's Mr. Dooley remarked, it was "hands across the water and into someone's pocket".