Most Americans are familiar with the famous Lincoln-Douglas Debates, though few may realize their true significance. In the debates, Abraham Lincoln discovered and exploited the fundamental contradiction in the antebellum Democratic Party between slavery and democracy. He so effectively articulated the contradiction that his argument ripped the Democratic Party in two, shattered the political unity of the Slave South, and, two years later, allowed the fledgling Republican Party to seize the presidency.

In 1858, Illinois Senator Stephen A. Douglas was up for reelection. Douglas, a Democrat, was a giant in American politics, known best for trying to strike a balance between “Free” and “Slave” States, first with the Compromise of 1850 and then with the Kansas-Nebraska Act of 1854. His opponent, by comparison, was a relative newcomer, a member of a party that had only been formed a few years earlier, and one that would likely fade into obscurity as quickly as it had emerged. Douglas claimed to stand for democracy, for the right of the men moving west to determine the conditions under which they lived, without interference from a domineering federal government back East. But the southerners who dominated his party also demanded the protection of slavery. Lincoln, in contrast, opposed the extension of slavery into the West. Douglas expected to crush both Abraham Lincoln and the Republican Party, but was sufficiently threatened by Lincoln’s attacks that he grudgingly agreed to a series of seven debates in different towns across the state.

In the first debate, Douglas preyed on the baser instincts of the crowd. He called Lincoln a “Black Republican” who would transform Illinois into a “free negro colony” and, through political equality, debase white men to the level of “negroes, Indians, and other inferior races.” He put Lincoln on the defensive, as Lincoln had to repeat that he believed that slavery was wrong and should not be extended, but admitted that he did not know what a post-slavery world would look like. Lincoln spent much of his time in the initial debate trying to address Douglas’s incisive attacks.

That dynamic changed in the second debate, when Lincoln stumbled upon an important question. Among other troubling ideas, the 1857 Dred Scott Decision had declared that Congress could not ban slavery in its Territories, Territories that in 1858 encompassed a vast amount of land in the West. But Douglas was an architect of “popular sovereignty,” or the idea that the men who settled new Territories ought to decide on the slavery question for themselves before admission as states. Which was it, Lincoln asked: did Douglas support the Supreme Court’s pro-slavery decision, or did he still believe that the people should have the ultimate say?

Lincoln had trapped Douglas in an impossible position. If Douglas supported the Court’s decision, he might lose support from moderate Democrats in Illinois, who saw that Dred Scott had thrown far too much power to slave-owners. But if he condemned the decision, he would alienate himself from hardline pro-slavery Democrats in the Deep South. He tried to sidestep the question, but Lincoln hammered him on it again and again. Douglas vacillated and declared the question to be nonsensical. Then, finally, in the debate in Freeport, Illinois, he claimed that the Dred Scott decision was a null point because Territorial legislature could ignore the Court’s decision by refusing to pass local measures, like Slave Codes, that would protect slave owners’ rights.

Fatally, Douglas had just declared that voters could nullify a Supreme Court decision, a position that directly contradicted Constitutional precedent. His attempt to wiggle out of Lincoln’s question is now known as the Freeport Doctrine, a half-measured attempt to appease both sides of Douglas’s party and gloss over the stark contradiction of the protection of slavery in a democratic republic.

With each moment of uncertainty and confusion, Lincoln grew bolder, his logic sharper. He pointed out that Douglas believed that slaves were property, and that the Constitution protected property. Therefore, Douglas had to protect slavery—and protect it everywhere, even against the wishes of the people—or reject the Constitution. Which was it, Lincoln persisted? Did Democrats fight for democracy or for slavery?

Followers of the debates became so enraptured by Lincoln’s logic that Douglas faced the question everywhere he travelled. The Sangamo Journal, a pro-Lincoln newspaper, noted that a crowd called on Douglas to clarify his position. “But Douglas… trembling with rage…. Declared that no gentleman would ask such a question, and declined to be interrogated further.” Douglas faced even greater pressure from his own party. Wealthy plantation owners trusted men like Douglas to protect slavery and open new lands for its expansion. Infuriated by Douglas’s waffling, they howled that the Dred Scott Decision had settled the matter—new Territories could not ban slavery—and that Douglas had betrayed them by suggesting that Territorial governments could override the Supreme Court’s decision. South Carolina’s Lancaster Ledger claimed that, “the highest motives of policy and self-protection must prompt the South the repudiate him.” Newspapers across the South ran similar stories.

In contrast, Lincoln displayed the cohesiveness of his party’s ideology. Republicans believed that slavery was wrong, and the United States government, with its founding principal of equality, ought to prevent its spread and set it down a course of ultimate extinction. In the final debate, Lincoln said:

“That is the real issue…. It is the eternal struggle between these two principles…. The one is the common right of humanity and the other is the divine right of kings…. No matter in what shape it comes, whether from the mouth of a king who seeks to bestride the people of his own nation and live by the fruit of their labor, or from one race of men as an apology for enslaving another race, it is the same tyrannical principle.”

While Lincoln lost the 1858 Senate race, he solidified his place as a key leader in the Republican Party and helped to fracture the Democratic Party. In the 1860 presidential election just two years later, Lincoln secured his party’s nomination, while the Democrats split between two factions. Douglas led one faction; John Breckinridge led another. Breckinridge declared that Douglas had not gone far enough in his defense of slavery and the Dred Scott Decision, and ran on a platform that directly rebuked Douglas’s Freeport Doctrine. The Breckinridge platform not only declared that Territories did not have the right to ban property in men, but also stated that the federal government must protect property—including property in slaves—if a local government did not. When the ballots were counted, Lincoln won less than forty percent of the vote, or about 1.8 million votes in total. In several southern states, he did not win a single vote, as they had refused to place him on the ballot. But Lincoln’s votes were enough to beat both Douglas and Breckinridge, who received just 1.3 million and 850 thousand votes, respectively, with another half million votes going to the status quo Constitutional Union Party. A united Democratic Party might have routed Lincoln and the Republicans.

Foiled at the ballot box, and realizing the collapse of their political power, slaveholders led their states out of the Union and formed the Confederacy. In their new constitution, they made sure to add provisions that would prevent the popular sovereignty dilemma from ever returning. According to Article I, Section 9.4, States in the Confederacy would not have the right to limit or eliminate slavery.