“The man who thinks that the perpetuity of slavery is essential to the existence of the Union, is unfit to be trusted. The deadliest enemy the Union has is slavery — in fact, its only enemy.”
Diary (5 June 1862)
Diary and Letters of Rutherford Birchard Hayes (1922 - 1926)
Context: These semi-traitors [Union generals who were not hostile to slavery] must be watched. — Let us be careful who become army leaders in the reorganized army at the end of this Rebellion. The man who thinks that the perpetuity of slavery is essential to the existence of the Union, is unfit to be trusted. The deadliest enemy the Union has is slavery — in fact, its only enemy.
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Rutherford B. Hayes 70
American politician, 19th President of the United States (i… 1822–1893Related quotes

Attributed to George Washington, John Bernard, Retrospections of America, 1797–1811, p. 91 (1887). This is from Bernard's account of a conversation he had with Washington in 1798. Reported as unverified in Respectfully Quoted: A Dictionary of Quotations (1989).
Posthumous attributions

“The Confederacy stands for slavery and the Union for freedom.”
Private conversation https://books.google.com/books?id=cpLsLWYhMLoC&printsec=frontcover&dq=%22not+a+man+shall+be+a+slave%22+%22Mcpherson%22&hl=en&sa=X&ved=0CC0Q6AEwAzgKahUKEwjiwOnYqoLIAhUIez4KHaTnDok#v=onepage&q=slavery&f=false (January 1862)
1860s

1860s, Letter to Horace Greeley (1862)

Source: 1860s, Speech at Hartford (1860)
Context: When they get ready to settle it, we hope they will let us know. Public opinion settles every question here, any policy to be permanent must have public opinion at the bottom, something in accordance with the philosophy of the human mind as it is. The property basis will have its weight. The love of property and a consciousness of right or wrong have conflicting places in our organization, which often make a man's course seem crooks, his conduct a riddle.

As quoted in Beverley Male (1982) Revolutionary Afghanistan: A Reappraisal, page 183

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

1860s, The Constitution of the United States: Is It Pro-Slavery or Anti-Slavery? (1860)
Context: My argument against the dissolution of the American Union is this. It would place the slave system more exclusively under the control of the slave-holding states, and withdraw it from the power in the northern states which is opposed to slavery. Slavery is essentially barbarous in its character. It, above all things else, dreads the presence of an advanced civilization. It flourishes best where it meets no reproving frowns, and hears no condemning voices. While in the Union it will meet with both. Its hope of life, in the last resort, is to get out of the Union. I am, therefore, for drawing the bond of the Union more completely under the power of the free states. What they most dread, that I most desire. I have much confidence in the instincts of the slaveholders. They see that the Constitution will afford slavery no protection when it shall cease to be administered by slaveholders. They see, moreover, that if there is once a will in the people of America to abolish slavery, this is no word, no syllable in the Constitution to forbid that result. They see that the Constitution has not saved slavery in Rhode Island, in Connecticut, in New York, or Pennsylvania; that the Free States have only added three to their original number. There were twelve Slave States at the beginning of the Government: there are fifteen now.

Diary entry (1820), as quoted in The Diary of John Quincy Adams (1951), by John Quincy Adams, Scribner's Sons, New York, p. 228-229 http://web.archive.org/web/20130703084250/http://home.nas.com/lopresti/ps6.htm