Marx became an adamant supporter of the Lincoln administration and the Union's cause during the course of the American Civil War. While Marx wasn't particularly known in America by the mid 1860s, he had cofounded the International Workingmen's Association by the fall of 1864, and drafted a letter to Lincoln on behalf of the organization following his reelection in the 1864 presidential election. The letter is praising of the abolitionist turn of Lincoln's executive authority and the progress of the war on the South. It reads:

"Sir:

We congratulate the American people upon your re-election by a large majority. If resistance to the Slave Power was the reserved watchword of your first election, the triumphant war cry of your re-election is Death to Slavery.

From the commencement of the titanic American strife the workingmen of Europe felt instinctively that the star-spangled banner carried the destiny of their class. The contest for the territories which opened the dire epopee, was it not to decide whether the virgin soil of immense tracts should be wedded to the labor of the emigrant or prostituted by the tramp of the slave driver?

When an oligarchy of 300,000 slaveholders dared to inscribe, for the first time in the annals of the world, "slavery" on the banner of Armed Revolt, when on the very spots where hardly a century ago the idea of one great Democratic Republic had first sprung up, whence the first Declaration of the Rights of Man was issued, and the first impulse given to the European revolution of the eighteenth century; when on those very spots counterrevolution, with systematic thoroughness, gloried in rescinding "the ideas entertained at the time of the formation of the old constitution", and maintained slavery to be "a beneficent institution", indeed, the old solution of the great problem of "the relation of capital to labor", and cynically proclaimed property in man "the cornerstone of the new edifice" — then the working classes of Europe understood at once, even before the fanatic partisanship of the upper classes for the Confederate gentry had given its dismal warning, that the slaveholders' rebellion was to sound the tocsin for a general holy crusade of property against labor, and that for the men of labor, with their hopes for the future, even their past conquests were at stake in that tremendous conflict on the other side of the Atlantic. Everywhere they bore therefore patiently the hardships imposed upon them by the cotton crisis, opposed enthusiastically the proslavery intervention of their betters — and, from most parts of Europe, contributed their quota of blood to the good cause.

While the workingmen, the true political powers of the North, allowed slavery to defile their own republic, while before the Negro, mastered and sold without his concurrence, they boasted it the highest prerogative of the white-skinned laborer to sell himself and choose his own master, they were unable to attain the true freedom of labor, or to support their European brethren in their struggle for emancipation; but this barrier to progress has been swept off by the red sea of civil war.

The workingmen of Europe feel sure that, as the American War of Independence initiated a new era of ascendancy for the middle class, so the American Antislavery War will do for the working classes. They consider it an earnest of the epoch to come that it fell to the lot of Abraham Lincoln, the single-minded son of the working class, to lead his country through the matchless struggle for the rescue of an enchained race and the reconstruction of a social world. "

The letter was received and transmitted to Washington by the American envoy in the UK, Charles Francis Adams (grandson of John Adams, son of John Quincy Adams). Adams' response to the letter was thus:

Sir:

I am directed to inform you that the address of the Central Council of your Association, which was duly transmitted through this Legation to the President of the United [States], has been received by him.

So far as the sentiments expressed by it are personal, they are accepted by him with a sincere and anxious desire that he may be able to prove himself not unworthy of the confidence which has been recently extended to him by his fellow citizens and by so many of the friends of humanity and progress throughout the world.

The Government of the United States has a clear consciousness that its policy neither is nor could be reactionary, but at the same time it adheres to the course which it adopted at the beginning, of abstaining everywhere from propagandism and unlawful intervention. It strives to do equal and exact justice to all states and to all men and it relies upon the beneficial results of that effort for support at home and for respect and good will throughout the world.

Nations do not exist for themselves alone, but to promote the welfare and happiness of mankind by benevolent intercourse and example. It is in this relation that the United States regard their cause in the present conflict with slavery, maintaining insurgence as the cause of human nature, and they derive new encouragements to persevere from the testimony of the workingmen of Europe that the national attitude is favored with their enlightened approval and earnest sympathies.

While many may try to write a more romantic or deep account of the relations of Abraham Lincoln and Karl Marx, this simple and short dialogue is the real extent of their communications that we can attest to. Lincoln died just a few months following the response of Adams. Marx was however consistent in his support for the political revolution of Reconstruction that continued following the end of the Civil War and the elevation of freed slaves as citizens and political actors. Lincoln certainly shared in his public rhetoric many sympathies to Marx's ideas of class struggle and alienation, as he voiced his belief that no man can truly be free while engaging in wage labor for a living, and even came out in support of striking and workers organizing during the New England Shoemakers Strike shortly before becoming President. However, it is a safe bet to say that Lincoln had probably never heard of Marx in any capacity before the 1864 letter was wired to him by Adams, and certainly never read any of his work. However, it's known that Lincoln went out of his way to read letters and dispatches to the White House daily, so it is very likely that he did personally read Marx's letter.

Interestingly though, the only American Marx mentions by name in his magnum opus Capital is Ben Wade, radical Republican senator from Ohio, alluding to a speech Wade gave on the inexorable feud between capital and labor that was becoming apparent by the late 1860s.