Douglas had a plethora of reasons that the public had secretly repudiated the Missouri Compromise, making his repeal of it in the Kansas–Nebraska Act no big deal. Lincoln began by grappling with the most potentially damaging one, the support he and other antislavery men now incensed with Douglas gave to the Wilmot Proviso. He first set out how the Missouri Compromise did not include any automatic provision to extend its line past the Louisiana Purchase. But he had still more to say about the Missouri Compromise and the great principle Douglas supposed it embodied:
Another fact showing the specific character of the Missouri law—showing that it intended no more than it expressed—showing that the line was not intended as a universal dividing line between free and slave territory, present and prospective—north of which slavery could never go—is the fact that by that very law, Missouri came in as a slave state, north of the line. If that law contained any prospective principle, the whole law must be looked to in order to ascertain what the principle was. And by this rule, the south could fairly contend that inasmuch as they got one slave state north of the line at the inception of the law, they have the right to have another given them north of it occasionally—now and then in the indefinite westward extension of the line. This demonstrates the absurdity of attempting to deduce a prospective principle from the Missouri Compromise line.
The line itself must rest on a great, deep truth of the universe that permanently divided slavery and freedom. After all, it reserved a section of land to freedom north of it and then reserved another section, also north of it, for slavery. If this encoded some principle of deep constitutional truth, then surely the North deserved a free state south of the line. Right, Stephen? The North accepted that compromise, so why not the South? Why did it break with the sacred pact and not suck up, say, a free Arkansas? Or, more on the point, why would it not accept a free California transgressing the line? That shouldn’t cause any problems, right?
Lincoln hammered it home, returning to Wilmot:
When we voted for the Wilmot Proviso, we were voting to keep slavery out of the whole Missouri [Mexican?] acquisition; and little did we think we were thereby voting, to let it into Nebraska, laying several hundred miles distant. When we voted against extending the Missouri line, little did we think we were voting to destroy the old line, then of near thirty years standing. To argue that we thus repudiated the Missouri Compromise is no less absurd than it would be to argue that because we have, so far, forborne to acquire Cuba, we have thereby, in principle, repudiated our former acquisitions, and determined to throw them out of the Union! No less absurd than it would be to say that because I may have refused to build an addition to my house, I thereby have decided to destroy the existing house! And if I catch you setting fire to my house, you will turn upon me and say I INSTRUCTED you to do it!
This struck at the weakest point of Douglas’ story: the transparent lie that everyone had secretly, but knowingly, repudiated and repealed the Missouri Compromise in 1850 and then, one supposes, forgot thereafter. Even Douglas’ liver probably couldn’t take the amount of lubrication required to honestly believe that. Everybody, even those who hated it, agreed in 1850 that the Missouri Compromise still stood.
And furthermore, antislavery men did not adhere to the Missouri Compromise only when it served them:
The most conclusive argument, however, that, while voting for the Wilmot Proviso, and while voting against the EXTENSION of the Missouri line, we never thought of disturbing the original Missouri Compromise, is found in the facts, that there was then, and still is, an unorganized tract of fine country, nearly as large as the state of Missouri, lying immediately west of Arkansas, and south of the Missouri Compromise line; and that we never attempted to prohibit slavery as to it. I wish particular attention to this. It adjoins the original Missouri Compromise line, by its northern boundary; and consequently is part of the country, into which, by implication, slavery was permitted to go, by that compromise. There it has lain open ever since, and there it still lies. And yet no effort has been made at any time to wrest it from the south. In all our struggles to prohibit slavery within our Mexican acquisitions, we never so much as lifted a finger to prohibit it, as to this tract. Is not this entirely conclusive that at all times, we have held the Missouri Compromise as a sacred thing; even when against ourselves, as well as when for us?
That territory, the future Oklahoma, remained Indian Country at the time of Lincoln’s speech. But some of the tribes there did practice slavery, and would fight for the Confederacy to save it, and no antislavery man proposed a slavery ban there. Back in 1820, such a movement had existed to keep slavery out of Arkansas. While slavery only explicitly got to keep Missouri, the men who drew the line knew that Arkansas would come into the Union eventually and knew they gave it up to slavery then. So the South really got two states for a compromise named after only one. It might have gotten still a third in years to come, possibly still more if the South endeavored to split up Texas.