“The government of the United States, under the conduct of the Democratic Party, has been all that time surrendering one plain and castle after another to slavery.”

Speech (1859)

1830s, Illinois House Journal (1837)

"The relation of slavery to a Republican form of government" https://archive.org/details/ASPC0005189300 (26 May 1858), New England Anti-Slavery Convention.
The relation of slavery to a Republican form of government (1858)

Source: 2000s, Wrong on Race: The Democratic Party's Buried Past (2008), p. xiii

United States v. Cruikshank, 92 U.S. 542 (1875)

Remarks on the Republican platform (1860)
Context: Whatever might have been the motive, few acts have ever been so barren of good, and so fruitful of evil. The contest has exasperated the public mind. North and South, and engendered feelings of distrust, and I may say hate, that I fear it will take years to wear away. The lamentable tragedy at Harper's Ferry is clearly traceable to this unfortunate controversy about slavery in Kansas.; and while the chief actor in this invasion has exhibited some traits of character which challenge our admiration, yet his fanatical zeal seems to have blinded his moral perceptions, and hurried him into an unlawful attack upon the lives of a peaceful and unoffending community in a sister State, with the evident intention of raising a servile insurrection, which no one can contemplate without horror; and few, I believe very few, can be found so indifferent to the consequences of his acts, or so blinded by fanatical zeal, as not to believe that he justly suffered the penalty of the law which he had violated.
The Whig party North and South having been completely broken up by the perpetration of this great wrong, and the subsequent attempt of the slave power, backed up by the President of the United States, to force slavery upon an unwilling people in Kansas, and by fraud and violence to make Kansas a slave State, a new phase was given to public affairs and to the parties in the country. The Democratic party became greatly divided and distracted by this outrage, and would also have been entirely demolished, if Southern States had not rallied to the support of that party. All the Southern States, with the exception of Maryland, having gone over to the support of the Democratic party, and the aggressions of the Southern propagandists of slavery in their attempt to send slavery everywhere, the Democratic party became essentially a Southern sectional party, inasmuch as very few public men South, of either party, could be sustained by their constituents in opposing these outrageous measures in Congress, and the frauds and rascalities committed in Kansas. All the compacts, resolutions, and agreements, to keep the peace, so recently made, having been broken, confidence was greatly impaired, indeed I may say entirely destroyed, in the Democratic party, and in this state of things a new party was formed, called the Repuulican Party, to resist the Democratic party in its new and alarming attitude of pro-slavery aggression.

"The Dirty Trickster" (2008)

As quoted in His Brother's Blood: Speeches and Writings, 1838–64 https://books.google.com/books?id=qMEv8DNXVbIC&pg=PA192 (2004), edited by William Frederick Moore and Jane Ann Moore, p. 192
1860s, Speech to the U.S. House of Representatives (April 1860)