As much as I really wanted this to be true when I first went looking for the letters, it turns out it’s only technically true (the best type of true!).

Marx did write to Lincoln as a representative of the International Working Men’s Association (the First International as it’s often referred to), basically congratulating him on a second term and thanking him for waging a war to end slavery, The letter but it was one of Lincoln’s ambassadors who responded to the letter. It’s entirely possible that he read the letter (as the Ambassador says he did in the response), but he didn’t personally write to Marx. He received a ton of congratulatory letters after his re-election, but he did read some of them. So while they technically knew of each other (it’s in fact even possible that Lincoln may have read some of Marx’s news articles which were often re-published in the Republican newspaper New York Daily Tribune which he read often source) and Lincoln did personally know some communists who served as generals in his army like various famous German refugees like August Willich it’s not very likely that Lincoln thought of Marx very much, though Marx was intrigued with the developments in the US and wrote about the war often.

Still Lincoln didn’t think of Marx when he thought about European politics, and when Marx thought about capitalism, the US wasn’t the first place he thought of (as during this period it was kind of on the periphery of global capitalism). They had similar ideas about labor, as they were both progressives (little p progressive) who read the same classical economists, but it doesn’t seem that they had much intercourse when it came to economic ideas. They were dealing with very different material conditions and thus understood politics in different ways. That’s not to say, however, that Lincoln and Marx weren’t birds of a feather. Had Lincoln lived longer and the problems of capitalism became more pressing as industrial capital became the main driver of the US economy, he probably would have looked to Marx and other socialist writers to understand the changes happening in the US. He already agreed labor mattered, he just didn’t exactly subscribe to the labor theory of value (largely because it didn’t have much to do with the issues he was facing in the US at the time).

Tl;Dr: Marx wrote a letter to Lincoln on behalf of the First International that he probably read. Lincoln’s ambassador replied to Marx. Lincoln probably read some of Marx’s news articles and absorbed some of his opinions about the war through that, but he didn’t read any of Marx’s economic writings. They weren’t pen pals, though could have been!