The leading Republican candidate for president in 1860 was Senator Seward of New York, distinguished by decades of experience in state and national government. But there was another candidate, relatively unknown nationally, but a recognized anti-slavery lawyer from Illinois. Some of his opponents delighted in calling Abraham Lincoln a “black Republican.”

Lincoln’s government experience consisted of one congressional term, preceded by four terms in the Illinois House of Representatives. Still, the dark horse candidate had earned a reputation for leadership of the Illinois anti-slavery coalition of former Whigs and free soil Democrats. In debates and major speeches of the 1850s, he had formulated a compelling, eloquent, often original case for prohibiting the extension of slavery to the territories. Invited to New York by city leaders not committed to Seward, Lincoln gave an extraordinary anti-slavery speech at Cooper Union on February 27, 1860.

After years in the national spotlight, Seward of New York had engendered much opposition even in his home base, but still he was the favorite. In addition to being a hard worker with a national following, he had an elite education, elite connections, political skills, a savvy political wife, a powerful political manager, and a well-oiled, well-financed political machine. Even Seward opponents, such as the mercurial New York Tribune editor Horace Greeley, thought Seward’s nomination all but certain at the coming Republican National Convention.

Nevertheless, on the third ballot, to an astonishing uproar, Lincoln broke through the deadlock in the convention at Chicago and became the party’s presidential candidate. The New York delegation was aghast. Only energetic efforts by Lincoln’s own managers brought the formidable Seward machine behind the dark horse candidate from the Midwest.

In the general election, Lincoln carried New York State, although he lost New York City by a 3-to-1 margin. The city as ever was a confused coalition of changing party allegiances. In October, before the election, fear of a Republican victory had caused many New York business leaders to try to create an anti-Republican fusion slate in New York. They worried about the commercial impact of a Republican anti-slavery victory, but they also anticipated an inconclusive national election, which would go to the House of Representatives. But some Democratic businessmen had gone over to the Republicans, as southern radicals preaching secession had scared them, and many also had reoriented themselves toward new opportunities in the Midwest.

After the election, further splits appeared. New York Republican businessmen issued an appeal to their fellow merchants: “… No true business man, it seems to us, who understands his own interests, can disregard the appeal made to him in this peculiar crisis of public affairs. We want peace and not panic, and there is only one way left now by which it can be secured.” Accommodation with the secessionists also appealed to widespread anti-black prejudice. Moreover, fear of defaults on Southern loans pushed some businesses and banks to try to settle debts. Many New York merchants and bankers tried to collect debts while simultaneously trying to repair rupturing relationships with the South.

In New York City, business leaders like John Dix formed the Union Committee of Fifteen to rally support for compromise with the South. In Washington, Senator Seward pressed for compromise. But the president-elect, unknown to the New York business leadership and the vast electorate, had firm thoughts about compromise. Lincoln’s six-year campaign from 1854 to 1860 brooked no compromise on the issue of slavery in the territories. Those who had read Lincoln’s speeches knew of his written and public pledges to give no ground to the expansion of America’s “peculiar institution. ” As for the fear of financial panic then inciting New York businessmen to compromise on slavery, Lincoln wrote privately: “I am not insensible to any commercial or financial depression that may exist; but nothing is to be gained by fawning around the ‘respectable scoundrels’ who got it up. …”

On the Democratic side in the 1860 presidential contest, Mayor Wood, who believed New York City’s economic interests were aligned with southern trade, had supported Southern compromise candidate John Breckinridge over Stephen Douglas, the nominee of the convention. During his most recent mayoral campaign, he had proclaimed: “The South is our best customer. She pays the best prices, and pays promptly,” and said that New York should “do nothing to estrange the South.”

Financier August Belmont, chairman of the northern Douglas wing of the Democratic Party, shared Wood’s economic interests. Belmont had a major stake in the threatened Southern cotton trade dominated by New Yorkers. Thus, the winter of 1860-1861 found New York business leaders feeling very vulnerable. They and Lincoln well remembered the Panic of 1857, which had shattered parts of the city’s economy.

As president-elect, Lincoln would hear that Mayor Wood had proposed in January 1861 that New York City secede from the Union, which Wood declared was doomed. Then, it was thought, New York could negotiate its own way between North and South.

Most New York merchants and bankers were more cautious. Perhaps compromise with the slaveholders, they thought, was the only way to avoid conflict. A prominent New York Republican businessman, Moses Grinnell, warned Lincoln that his New York supporters “shudder at the thought of risking the advantages of the Union, in all its integrity, on the territorial question.” Lincoln was urged to compromise his Republican platform pledge to prohibit the extension of slavery in the territories, but Lincoln did not waver.

President-elect Lincoln, on the way to Washington, arrived in New York City on February 19, 1861. He was not enthusiastically received, but rather was greeted, as the poet Walt Whitman wrote, by a “sulky, unbroken silence such as certainly never before characterized so great a New York crowd.”

At an appearance at City Hall, Lincoln focused on the central issue alone, embracing the Union as a ship whose helm he was ready to take: “There is nothing that could ever bring me willingly to consent to the destruction of this Union, under which not only the great commercial city of New York, but the whole country, acquired its greatness, except it be for the purpose for which the Union itself was formed. …” Lincoln emphasized that the Declaration of Independence had set forth the purpose for which the Union had been created, “that all men are created equal” and “endowed by their Creator” with the inalienable right to life and liberty. For Lincoln to embrace the interests of finance and commerce — over the Constitution, over the Union, over the Declaration of Independence — was unacceptable.

As time would tell, Lincoln intended, by all constitutional and necessary means, to preserve the Union. His oath to defend the Constitution, and thus the Union, was, as he later remarked, an oath “registered in Heaven.”

During the next four years, New Yorkers, depending upon their opinions and circumstances, would both praise and denounce his adherence to the oath. But most did not clearly understand his statecraft, nor did they know his relentless will and purpose — until he was assassinated. On April 15, 1865, Mayor Gunther notified New Yorkers: “The death of the President may well excite your profound grief and amazement. I respectfully recommend that business be suspended, and that a public mourning for the departed Chief Magistrate be observed throughout the city.” The business of New York had come to a stop.

In that moment — New York, the North, and the South — all sensed the Union, on President Lincoln’s watch, had been preserved — and that the abolition of slavery by the 13th Amendment had been assured.

Mr. Lehrman, co-founder of the Gilder-Lehrman Institute of American history, writes for the Lincoln Institute, which sponsors the Web site mrlincolnandnewyork.org.