This letter was sent from Abraham Lincoln to Jesse A. Pickrell, a farmer living near Mechanicsburg, Illinois, and friend of Lincoln's.
During the presidential election of 1856, Abraham Lincoln had put his support behind John C. Fremont, the Republican candidate. Fremont's competition were the Democratic candidate James Buchanan and the American Party (also known as the Know-Nothing party) candidate Millard Fillmore. Lincoln and the Republicans were against the pro-slavery and pro-popular sovereignty platforms of Buchanan and the Democrats. In order to prevent a Buchanan victory, Lincoln developed a form letter he sent to Fillmore supporters in Sangamon County, Illinois, to try persuade them to vote for Fremont. He pointed out that voting for Fillmore would result in splitting the vote and allowing Buchanan to take the state, lessening the chance of either of the other candidates winning. Ultimately, Buchanan received 105,528 votes in Illinois, compared to Fremont's 96,275 and Fillmore's 37,531, winning the state as well as the presidency.
In his letter to Jesse A. Pickrell on September 15, 1856, Lincoln requested that Pickrell quietly assemble a list of fifteen or twenty Fillmore supporters in the area near Springfield, and also asked that he get their addresses - presumably for Lincoln to send his form letter to these voters.
A transcription of this note can be found in The Collected Works of Abraham Lincoln (edited by Roy P. Basler et al.) in the second supplement (also called Volume 11) on page 12. An online version of the transcription can be found in the online Collected Works: https://quod.lib.umich.edu/l/lincoln/lincoln2/1:400?rgn=div1;view=fulltext.
The letter was acquired by the University Library in 1966, with support from the Dr. Harlan Horner Estate.