‘I have never had a feeling politically that did not spring from the sentiments embodied in the Declaration of Independence,” Abraham Lincoln told a crowd in front of Independence Hall on the morning of Feb. 22, 1861. “I have often pondered over the dangers which were incurred by the men who assembled here and adopted that Declaration of Independence — I have pondered over the toils that were endured by the officers and soldiers of the army, who achieved that independence.”

The theme that Lincoln pursued early that morning, just two weeks before his inauguration, was one he had elaborated for nearly seven years — and would return to as president.

A journalist who covered Lincoln from 1854 to 1860 noted that when the Illinois lawyer spoke of the Declaration, “He rose to impassioned eloquence . . . as his soul was inspired by the thought of human right and Divine justice.” Lincoln’s own law partner called the Declaration “his greatest inspiration.”

In 1861, on the anniversary of the birth of President George Washington, President-elect Lincoln evoked the birth of the Republic. “I have often inquired of myself, what great principle or idea it was that kept this Confederacy so long together,” he told the enthusiastic crowd.

“It was not the mere matter of the separation of the colonies from the mother land; but something in that Declaration giving liberty, not alone to the people of this country, but hope to the world for all future time. It was that which gave promise that in due time the weights should be lifted from the shoulders of all men, and that all should have an equal chance. This is the sentiment embodied in that Declaration of Independence.”

After the Kansas-Nebraska Act became law in 1854, permitting slavery to spread into the northern Louisiana Purchase, Lincoln campaigned throughout Illinois for repeal of the new law. Time and again, he explained the abiding importance to America of the Declaration and its equality principle: “All men are created equal.”

He defended this principle of the American founding against a vast throng in the North and South who maintained that it did not apply to black slaves brought over from Africa. He presented historical evidence that the Founders had intended that slavery in America should be put in the “course of ultimate extinction.” (Indeed, Lincoln’s hero, George Washington, had used his will and testament to free his own slaves.)

By reversing the prohibition on slavery in the northern section of the Louisiana Purchase, the Kansas-Nebraska Act threatened to extend slavery, perhaps throughout America. And the Supreme Court’s ruling in the Dred Scott case threatened to eliminate all barriers to slavery and perhaps make it a lawful national institution. Lincoln feared that the two developments would render the Declaration of Independence a “glittering generality.”

In Dred Scott, the Supreme Court held that the Declaration’s equality clause did not apply to black Americans — that black Americans could never be American citizens under the Constitution. Lincoln, as president, maintained that under the Declaration and the Constitution, black Americans could and should be citizens.

For Lincoln, the principles of the Founding, namely the Declaration of Independence, were too fundamental to be ignored by Americans. “Most governments have been based practically on the denial of the equal rights of men . . . Ours began by affirming those rights,” Lincoln wrote. “Possibly so, said we; and by your system, you would always keep them ignorant and vicious. We propose to give to all a chance, and we expect the weak to grow stronger, the ignorant, wiser; and all better, and happier together.”

For Lincoln, the American Union was founded by the Declaration of Independence. Yet the promise of the Declaration could only be fulfilled when Lincoln’s “oft-expressed personal wish that all men everywhere could be free” became the constitutional law of America.

With the 13th Amendment of 1865, slavery was abolished, and the promise could ultimately be fulfilled.

Lewis E. Lehrman is co-founder of the Gilder-Lehrman Institute of American History and author of “Lincoln at Peoria: The Turning Point.”