In a break with tradition, Donald Trump plans to assume a prominent role in the Independence Day celebration in Washington tomorrow. For months, Trump has promised a “major fireworks display, entertainment and an address by your favourite president, me”!

Democrats have expressed concern that his speech from the Lincoln Memorial will be little more than a campaign event. Given Trump’s penchant for self-aggrandisement, it seems reasonable to expect that his thoughts on the meaning of the Fourth of July will focus on the success of his presidency and the unfairness of the media’s treatment of him.

Moreover, his policies on immigration and healthcare, and his willingness to exploit misogyny and bigotry, all of which reflect a limited vision of human liberty, suggest he is unlikely to invoke America’s founding documents or sacred creeds, as did the President whose monument will be the backdrop for his speech. The sharp contrasts between the two men will be on full display as the nation celebrates another Independence Day, uncertain about the fate of democracy at home and abroad.

In another moment of such uncertainty, Abraham Lincoln found inspiration in the nation’s founding principles. On the evening of July 7, 1863, with the third Independence Day of the Civil War just passed, he responded to a crowd of serenaders who had gathered outside the White House to celebrate recent Union victories. The rebellion, he observed, was “an effort to overthrow the principle that all men are created equal”. It was, therefore, fitting that the enemies of the Declaration of Independence surrendered at Vicksburg, Mississippi, and retreated from Gettysburg, Pennsylvania, on Independence Day.

Lincoln had not prepared a speech, as he quickly acknowledged in concluding his brief remarks. Given their impromptu nature, it shouldn’t surprise us that Lincoln had turned to what was, for him, a familiar theme — and one to which he would famously return four months later at Gettysburg. For more than a decade, he had argued that nothing was more important to the survival of the nation than living up to the proposition that all men are created equal.

In 1852, Lincoln’s brief political career — one term in Congress — had seemed over. Even so, he openly fretted about a new political development: “an increasing number of men” were beginning to assail the Declaration of Independence “for the sake of perpetuating slavery”. This danger became far more urgent two years later when Congress passed the Kansas-Nebraska Act, which repealed the part of the Missouri Compromise of 1820 that had prohibited slavery in the remaining Louisiana Purchase territory north of 36/30 latitude. Stephen Douglas, Lincoln’s rival in Illinois politics and the Bill’s author, intended that the people of Kansas and Nebraska should be “perfectly free” to allow or prohibit the institution of slavery, a policy euphemistically known as popular sovereignty.

Douglas’s Bill alarmed Lincoln enough to bring him back into politics. By relinquishing control over slavery’s expansion to territorial settlers, popular sovereignty fostered and reflected national indifference to the future of human servitude, which Lincoln denounced because “it assumes that there can be moral right in the enslaving of one man by another”. This assumption was clearly incompatible with the Declaration. He was especially appalled by one of Douglas’s supporters, John Pettit of Indiana, who proclaimed on the floor of the US Senate that the Declaration of Independence was “a self-evident lie”.

Time and again over the next six years, as he helped build the Republican Party, Lincoln warned his audiences that failing to uphold the principle that all men are endowed with the rights to life, liberty and the pursuit of happiness would destroy the foundation of everyone’s liberty, regardless of race: “I should like to know, taking this old Declaration of Independence, which declares that all men are equal upon principle, and making exceptions to it — where will it stop? If one man says it does not mean a Negro, why not another say it does not mean some other man?”

Although Douglas never called the Declaration a “self-evident lie”, his official indifference to the spread of slavery forced him to limit its meaning. He repeatedly appealed to white supremacy — to which nearly all white Americans, North and South, were devoted in the 1850s — proclaiming his belief that the signers of the Declaration “referred to the white race alone, and not to the African, when they declared all men to have been created equal”. In fact, Douglas was certain that the signers meant to exclude all “inferior races”, such as Native Americans, the Chinese and the Japanese. “They were speaking,” he insisted, “of British subjects on this continent being equal to British subjects born and residing in Great Britain.”

Lincoln accused Douglas of making a “mangled ruin” of the nation’s founding document. As a mere statement of equality between British subjects in America and those across the pond, it was stripped of all relevance to succeeding generations of Americans, many of whom, he noted, were not descendants of British colonists. “Are you really willing that the Declaration shall be thus frittered away? — thus left no more at most, than an interesting memorial of the dead past? Thus shorn of its vitality, and practical value; and left without the germ or even the suggestion of the individual rights of man in it?”

Lincoln understood that this was more than a philosophical question. Undermining the Declaration was dangerous, because the rationales for slavery were not inherently racial. Anyone could fall victim to the argument that they were “inferior”, that they would be better off enslaved, that their enslavement served the interests of society or that their subjugation was justified by history and religion. Lincoln made this point by comparing proslavery arguments to those that sustained divine-right monarchy. As he explained in his almost-successful Senate campaign against Douglas in 1858, stealing the fruits of another person’s labour was always “the same tyrannical principle”, regardless of whether the oppressor was a king or a race of men.

Denying or qualifying the Declaration for slavery’s benefit would therefore “delight a convocation of crowned heads plotting against the people”, he wrote in a public letter commemorating Jefferson’s birthday the next year. It was America’s great fortune that the Declaration’s author had introduced “into a merely revolutionary document, an abstract truth, applicable to all men and all times, and so to embalm it there, that today, and in all coming days, it shall be a rebuke and a stumbling block to the very harbingers of reappearing tyranny and oppression”.

Throughout his career, Lincoln maintained steadfastly that the principles of the Declaration required the American people to repudiate slavery. This did not require its immediate abolition in the South — nor did he favour equality of the races. Prohibiting the expansion of slavery, thus keeping it “in the course of ultimate extinction”, would signify that the public considered it a necessary evil, to be tolerated temporarily as an aberration in a nation still dedicated to the proposition that all men are created equal.

Commitment to this principle and recognition that slavery was a moral evil — and that those like Robert E. Lee who fought for the subjugation of their fellow men were traitors — separates Lincoln from the sitting president.

As Trump stands tomorrow at the Lincoln Memorial, it will be a study in contrasts: a president suspicious of equality and democracy standing before the image of one who worked to ensure that his nation would some day live up to its founding principles, and who sacrificed his life to that cause. Trump will be unlikely to spend much time reflecting on that contrast, but the rest of us should.

• Jeremy Tewell is the author of A Self-Evident Lie: Southern Slavery and the Threat to American Freedom