“In all this I can see but the doom of slavery. The North do not want, nor will they want, to interfere with the institution. But they will refuse for all time to give it protection unless the South shall return soon to their allegiance.”

Source: Surfacing

Source: Letter to My Daughter

Speech in the Senate, February 14, 1850, in response to a speech by Senator Henry S. Foote of Mississippi, who had "lectured" Clay on the allegiance which he owed to the South as a senator from a Southern state. From The Life, Correspondence, and Speeches of Henry Clay (Vol. 3); ed. Calvin Colton: A. S. Barnes & Co., 1857.

Source: Look Away!: A History of the Confederate States of America (2002), pp. 97–98

2010s, Confederation Again (July 2018)

Johannesburg, South Africa, 2 May, 1972
1970s

First Debate with Stephen Douglas in the Lincoln-Douglas debates http://www.bartleby.com/251/ of the 1858 campaign for the U.S. Senate, at Ottawa, Illinois (21 August 1858). Lincoln later quoted himself and repeated this statement in his first Inaugural Address (4 March 1861) to emphasize that any acts of secession were over-reactions to his election. During the war which followed his election he eventually declared the Emancipation Proclamation, freeing the slaves in those states in rebellion against the union, arguably as a war measure rather than as an entirely political or moral initiative.
1850s, Lincoln–Douglas debates (1858)

1860s, Interview with Alexander W. Randall and Joseph T. Mills (1864)
Context: There have been men who have proposed to me to return to slavery the black warriors of Port Hudson and Olustee to their masters to conciliate the South. I should be damned in time and in eternity for so doing. The world shall know that I will keep my faith to friends and enemies, come what will.

Source: Look Away!: A History of the Confederate States of America (2002), p. 3