Tariffs were vital to the US economic expansion after the Civil War, without them the US would have been flooded with cheaper British goods during the second half of the 19th Century and American factories would not have survived. From 1865 to 1914 the average tariff rate was over 40%. As much as the USA champions free trade, that only started after WWI when the USA had become the world's biggest economy.
en.wikipedia.org
Indeed tariffs helped to fund internal improvements and pay off the US war debt, and protected then-young US manufacturing from outside competition, but the issue became very different later.
The most important reason behind late 19th century American protectionism was likely not so much the protection of infant industries, since the 1880s the gradually became a net exporter of industrial goods. There had consistently been significant pressures to lower the tariffs, but the more hardcore classical republicans resisted it on ideological grounds, even though it ran against their economic interests at times. Even within the then-protectionist GOP many sections realized the counterproductiveness of tariffs. (William Taft, for example)
Tariffs were important to the US because it was one of the only sources of government revenue back then, as the US constitution initially did not provide the federal government a viable way for the federal government to collect income taxes. Sixteenth Amendment became a heated debate even before Wilson, with republican progressives and democratic populists wishing to replace tariffs with income taxes, which are a more stable and equitable source of revenue. Rich people opposed free trade not because they really needed any protection from outside competition, they mostly created excuses so they don’t have to take over the tax burden.
With the eventual adoption of an income tax, of course the importance of tariffs decreased, that doesn’t mean the US has become a champion of free trade, in contrast to what the media tells people, the US still launches WTO complaints and antidumping investigations far more frequently than everyone else, the US runs a trade deficit now simply because its biggest export is the greenback.
It’s a very interesting story overall, and I can’t wait to see if/how Victoria 3 will represent this in game.