I Shall not pause to consider whether my Opinion will be popular or unpopular with the Slave Holders, or Slave Traders, in the Northern the Middle, the Southern, or the Western, States—I respect all those who are necessarily subjected to this Evil.—But Negro Slavery is an evil of Colossal Magnitude. … I am therefore utterly averse to the admission of Slavery into the Missouri Territory, and heartily wish that every Constitutional measure may be adopted for the preservation of it.

Letter to William Tudor, Jr., 20 November 1819. Partially quoted in Founding Brothers : The Revolutionary Generation (2000) by Joseph J. Ellis, p. 240