“I hope none who hear me will confound this expression of mine with advocacy of the right of a state to remain in the Union, and to disregard its constitutional obligations by the nullification of the law. Such is not my theory. Nullification and secession, so often confounded, are indeed antagonistic principles. Nullification is a remedy which it is sought to apply within the Union, and against the agent of states. It is only to be justified when the agent has violated his constitutional obligation, and a state, assuming to judge for itself, denies the right of the agent thus to act, and appeals to the other states of the Union for a decision; but when the states themselves, and when the people of the states, have so acted as to convince us that they will not regard our constitutional rights, then, and then for the first time, arises the doctrine of secession in its practical application.”

1860s, Fourth of July Address to Congress (1861)

On the Irrepressible Conflict (1858)
Context: The Union is a confederation of States. But in another aspect the United States constitute only one nation. Increase of population, which is filling the States out to their very borders, together with a new and extended network of railroads and other avenues, and an internal commerce which daily becomes more intimate, is rapidly bringing the States into a higher and more perfect social unity or consolidation. Thus, these antagonistic systems are continually coming into closer contact, and collision results.
Shall I tell you what this collision means? They who think that it is accidental, unnecessary, the work of interested or fanatical agitators, and therefore ephemeral, mistake the case altogether. It is an irrepressible conflict between opposing and enduring forces, and it means that the United States must and will, sooner or later, become either entirely a slaveholding nation, or entirely a free-labor nation.

Source: The Characteristics of the Present Age (1806), p. 186

"Outline" notes (September 1829), in The Writings of James Madison (1910) by Gaillard Hunt, Vol. 9, p. 357. Inscribed in the Madison Memorial Hall, Library of Congress James Madison Memorial Building.
1820s

Texas v. White, 7 Wallace, 725 (1869)

1910s, Address to the Knights of Columbus (1915)
Context: One of the most important things to secure for him is the right to hold and to express the religious views that best meet his own soul needs. Any political movement directed against anybody of our fellow- citizens because of their religious creed is a grave offense against American principles and American institutions. It is a wicked thing either to support or to oppose a man because of the creed he professes. This applies to Jew and Gentile, to Catholic and Protestant, and to the man who would be regarded as unorthodox by all of them alike. Political movements directed against men because of their religious belief, and intended to prevent men of that creed from holding office, have never accomplished anything but harm. This was true in the days of the ‘Know-Nothing’ and Native-American parties in the middle of the last century; and it is just as true to-day. Such a movement directly contravenes the spirit of the Constitution itself. Washington and his associates believed that it was essential to the existence of this Republic that there should never be any union of Church and State; and such union is partially accomplished wherever a given creed is aided by the State or when any public servant is elected or defeated because of his creed. The Constitution explicitly forbids the requiring of any religious test as a qualification for holding office. To impose such a test by popular vote is as bad as to impose it by law. To vote either for or against a man because of his creed is to impose upon him a religious test and is a clear violation of the spirit of the Constitution.

5. U.S. (1 Cranch) 137
Marbury v. Madison (1803)

1920s, Freedom and its Obligations (1924)

2000s, The Real Abraham Lincoln: A Debate (2002), Rebuttal