I can quote at least one. Too bad for Wlat that he's not exactly the kind of guy that most sane people want to have on their side...
"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." - Karl Marx, letter to Abraham Lincoln, Nov. 29, 1864
Dr. Marx then brought up the report of the subcommittee, also a draft of the address which had been drawn up for presentation to the people of America congratulating them on their having re-elected Abraham Lincoln as President. The address is as follows and was unanimously agreed to.
The minutes continue:
A long discussion then took place as to the mode of presenting the address and the propriety of having a M.P. with the deputation; this was strongly opposed by many members, who said workingmen should rely on themselves and not seek for extraneous aid.... It was then proposed... and carried unanimously. The secretary correspond with the United States Minister asking to appoint a time for receiving the deputation, such deputation to consist of the members of the Central Council.
So rather than a personal letter to Lincoln from Marx as you insinuate, it is an address from the International Workers Association (IWA)General Counsel to the American people, delivered through our London embassy.
Nice try though...