In 1860, two of the four candidates for president of the United States came from Illinois. The Republican, Abraham Lincoln, stopped all public communication in March after completing a tour through New York and New England. His Illinois Democratic opponent, Stephen A. Douglas, spoke out extensively. The quiet Republican won the presidency, maintaining a disciplined silence until he left Springfield for Washington on February 11, 1861. The Vermont-born Democrat, Senator Douglas, had broken political tradition by campaigning almost full time. It was his third try for the presidency.

The Kentucky-born Lincoln had only a little national political experience. He had campaigned in the Midwest during 1859 — explaining the Republican Party’s unswerving opposition to the spread of slavery, and its repudiation of the doctrine of popular sovereignty espoused by Senator Douglas. This doctrine advocated giving voters in American territories the choice of taking slaves or not. Moral indifference to slavery grounded the idea of popular sovereignty.

Lincoln and Douglas often had debated the slavery issue in Illinois after the passage of the Kansas Nebraska Act in 1854. This Douglas-led Congressional Act repealed the provision of the Missouri Compromise of 1820 prohibiting slavery north of the 36º 30′ parallel of the Louisiana Purchase. Lincoln opposed the extension of America’s “peculiar institution” into the Kansas Nebraska Territory.

In debates at Springfield and Peoria in the fall of 1854, Lincoln furthermore rejected the argument of Douglas that the equality proposition of the Declaration of Independence did not apply in principle to blacks as well as whites. At Peoria on the evening of October 16, 1854, Lincoln gave a three-hour reply to a Douglas speech on popular sovereignty. In a single sentence, Lincoln grounded his own moral opposition to slavery: “Slavery is founded in the selfishness of man’s nature — opposition to it in his love of justice.”

Lincoln led the campaign in Illinois against the Kansas Nebraska Act and the extension of slavery during the fall of 1854. But he lost the vote for the U.S. Senate seat, in the state legislature, in early 1855. Lincoln’s intellectual battle with Douglas would be renewed by the Dred Scott decision of the U.S. Supreme Court in March 1857. Chief Justice Roger B. Taney denied that blacks could be American citizens. Moreover, Taney declared that American negroes were not meant to be included in the Declaration of Independence. Lincoln argued that Taney’s decision improperly reversed the universal principle of America’s founding charter — the Declaration of Independence, that “all men are created equal.”

A year later, Lincoln initiated his campaign to unseat Senator Douglas by arguing that there was a “conspiracy” among Taney, Douglas, and two Democratic presidents to make slavery into a national institution. At the debate with Senator Douglas in Alton on October 15, 1858, Lincoln summed up the threat to the Union: “What has ever threatened our liberty and property save and except this institution of Slavery?” Lincoln led the Republican Party as it carried the popular vote in the state legislative elections during the fall of 1858. Again, in the state legislature, he lost the U.S. Senate seat.

During 1859, he sustained his anti-slavery campaign, which led to his speech on February 27, 1860 at Cooper Union in New York City. There, Lincoln spelled out the history of the Founders’ actions to restrict slavery — to put it “in the course of ultimate extinction.” Lincoln now addressed the threat of secession: “Neither let us be slandered from our duty by false accusations against us, nor frightened from it by menaces of destruction to the government nor of dungeons to ourselves. Let us have faith that right makes might, and in that faith, let us, to the end, dare to do our duty as we understand it.”

Throughout the 1850s, Lincoln and Douglas had advocated different positions on slavery, based on different conceptions of the Declaration of Independence. But the Confederate attack on Fort Sumter on April 12, 1861 brought the long-time antagonists together in an effort to defend the Union. After meeting with President Lincoln, Senator Douglas delivered a statement to the press saying that although he “was unalterably opposed to the administration on all its political issues, he was prepared to sustain the President in the exercise of all his constitutional functions to preserve the Union, and maintain the government, and defend the Federal Capitol. A firm policy and prompt action was necessary.”

On April 25, 1861, Douglas, having returned to Illinois, addressed an overflow crowd at the Illinois State Capitol — where he and Lincoln had faced off in October 1854. “Hostile armies are now marching upon the Federal Capitol with the view of planting a revolutionary flag upon its domes; seizing the national archives; taking captive the President,” said Douglas. “The boast has gone forth by the Secretary of War of this revolutionary [Confederate] government that on the 1st of May the revolutionary flag shall float from the wall of the capitol in Washington, and that on July 4th the revolutionary army shall hold possession of the Hall of Independence in Philadelphia. The simple question presented to us is whether we shall wait for the enemy to carry out his boast of making war upon our soil, or whether we shall rush as one man to the defense of our government and its capitol.”

By July 4, 1861, Douglas was dead. Congress had then reassembled to hear a special message from President Lincoln at the U.S. Capitol. There, the commander-in-chief declared that civil war “embraces more than the fate of these United States. It presents to the whole family of man, the question, whether a constitutional republic, or a democracy — a government of the people, by the same people — can, or cannot, maintain its territorial integrity, against its own domestic foes.” For America, the victory of Union armies answered this grave question.

Lincoln never won the Senate seat he sought. Douglas never reached the presidency to which he aspired. Through war the Union, which both revered, would survive. By Union victory, slavery would be abolished in 1865, with the Thirteenth Amendment to the Constitution of the United States.

Mr. Lehrman, co-chairman of the Gilder Lehrman Institute of American History, gilderlehrman.org, is the author of the forthcoming book, “Lincoln at Peoria: The Turning Point.”