On April 23, 1860, 148 years ago, the Democratic National Convention opened in Charleston, S.C. There, the Democratic Party disintegrated over the possible nomination of Stephen Douglas, a senator of Illinois.

For a decade, Douglas had been a dominant and charismatic figure in the Democratic Party. He was, wrote historian Bruce Catton, “a man about whom no one could be indifferent.” Douglas repeatedly proclaimed his own indifference to the extension of slavery in order to please the South. Nevertheless, slaveholding Democratic leaders rejected him and his popular sovereignty — a doctrine that would allow each territory to decide for or against slavery.

Douglas saw himself as a unifying figure, but southern slaveholders believed him a divisive one, thinking his indifference to their “peculiar institution” a smokescreen by which to block slavery in the Kansas territory. The Democratic supporters of Douglas argued in vain that only the senator from Illinois could save the Democrats from a Republican victory.

Democrats first divided on a platform statement — whether Congress or the Supreme Court was the ultimate arbiter on slavery in the territories. The anti-Douglas forces walked out of the convention, led by delegates from seven southern states.

Still, Douglas could not win the nomination, because Democratic Party rules required a two-thirds majority of the delegates. The Massachusetts chairman ruled that Douglas needed two-thirds of all delegates, not two-thirds of those who stayed at the convention. The remnant opposed to Douglas guaranteed that he could not pass the necessary threshold. Some Democrats preferred to wreck their party rather than to allow Douglas a chance at the presidential oath. After three days of indecisive balloting, the Democratic convention decided to reconvene in Baltimore.

Two weeks thereafter, delegates to the Republican National Convention gathered in Chicago. The presumptive favorite for the nomination was a senator of New York, William Henry Seward. However, there were serious doubts about Seward among Republican leaders in Illinois, Indiana, Pennsylvania, and New Jersey. They wondered if Seward could carry their states. Their opposition prevented Seward from winning on the first ballot.

During the next two ballots, votes shifted to a former congressman of Illinois, Abraham Lincoln. Douglas and Lincoln had been Illinois political rivals for almost three decades — Douglas sympathetic to slaveholder interests, Lincoln the anti-slavery leader in Illinois.

When Lincoln defeated Seward on the third ballot, pandemonium erupted in the convention “Wigwam” at Chicago. Lincoln had seen the opening for Republicans before the February 1860 Cooper Union speech in New York: “The southern Democrats will refuse to support [Douglas] for president and he cannot hope to win without them. Four years ago, I could see no prospect of the Republican party winning before 1864 at the earliest; now I believe that with the right candidate on the right platform we shall win an easy victory in November.”

Geography, slavery, and race seemed to be destiny for the parties in 1860. Republicans chose Chicago in 1860 because of its pivotal importance in the booming old northwest. The selection of Charleston, S.C., by the Democrats recognized the party’s base in the South. But there was probably no less hospitable place for the nomination of Douglas than Charleston. On the other hand, Chicago provided the home-state for a surprise nomination of Illinois’s favorite Republican son.

Northern Democrats would nominate Douglas; southern Democrats would nominate Vice President Breckinridge; the Constitutional Union Party (primarily old-line Whigs) would nominate Tennessean John Bell.

Douglas campaigned across the country — for the first time in the history of American presidential politics. But all the impassioned oratory of Douglas could not overcome the split in the dominant Democratic Party. Douglas, the famous Illinois senator, would lose to attorney Lincoln, at that time a private citizen from Springfield, Ill.

As Lincoln had declared when he launched his anti-slavery campaign in 1858 against Douglas, “A house divided against itself cannot stand.”

Mr. Lehrman, the author of “Lincoln at Peoria — Getting Right with the Declaration of Independence,” is a co-founder of the Gilder Lehrman Institute of American History.