In the spring of 1848, Horace Mann was elected to the United States Congress as a Whig, to fill the vacancy caused by the death of John Quincy Adams.  

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His first speech to Congress advocated for the right and duty of the government to exclude slavery from the territories, saying “I think the country is to experience serious times.  Interference with slavery will excite civil commotion in the South.  But it is best to interfere.  Now is the time to see whether the Union is a rope of sand or a band of steel.”  He said, “I consider no evil as great as slavery.” 

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He was re-elected to Congress, running as an independent anti-slavery candidate (defeating Daniel Webster) and served from April 1848 until March 1953.