The Walker Tariff remained in place until 1857, when a nonpartisan coalition lowered them again with the Tariff of 1857 to 18%. This was in response to the British repeal of their protectionist "Corn Laws".
The Democrats in Congress, dominated by Southern Democrats, wrote and passed the tariff laws in the 1830s, 1840s, and 1850s, and kept reducing rates, so that the 1857 rates were down to about 15%, a move that boosted trade so overwhelmingly that revenues actually increased from just over $20 million in 1840 to more than $80 million by 1856. The South had almost no complaints but the low rates angered many Northern industrialists and factory workers, especially in Pennsylvania, who demanded protection for their growing iron industry. The Republican Party replaced the Whigs in 1854 and also favored high tariffs to stimulate industrial growth; it was part of the 1860 Republican platform.
The Morrill Tariff significantly raising tariff rates became possible only after the Southern Senators walked out of Congress when their states left the Union, leaving a Republican majority. It was signed by Democratic President James Buchanan in early March 1861 shortly before President Abraham Lincoln took office. Pennsylvania iron mills and New England woolen mills mobilized businessmen and workers to call for high tariffs, but Republican merchants wanted low tariffs. The high tariff advocates lost in 1857, but stepped up their campaign by blaming the economic recession of 1857 on the lower rates. Economist Henry Charles Carey of Philadelphia was the most outspoken advocate, along with Horace Greeley and his influential newspaper, the New York Tribune. Increases were finally enacted in February 1861 after Southerners resigned their seats in Congress on the eve of the Civil War.https://en.wikipedia.org/wiki/Tariffs_in_United_States_history#Low_tariff_of_1857