First-term incumbent Know Nothing mayor Levi Boone did not seek reelection. His tenure had been very unpopular, and his anti-immigrant policies had led to a strong blowback from Chicago's immigrant community. Taking place several years prior to the start of the American Civil War, the election was shaped by the tenuous national debate surrounding slavery. The campaign was particularly shaped by debate surrounding the Kansas–Nebraska Act. The debate was so central to the election that rather than affiliate with traditional political parties, both candidates for mayor instead ran under the banner of "Pro-Nebraska" and Anti-Nebraska. Democratic US SenatorStephen A. Douglas, an author of the Nebraska-Kansas Act who was seeking the 1856 Democratic presidential nomination and who would be up for reelection as a senator in two years, sought to bolster his electoral prospects by utilizing the Chicago mayoral election to illustrate popular support in Illinois for his stance on slavery. Douglas strongly backed Dyer and sought to frame the mayoral race as a referendum about his stance on slavery. He and his allies had much to gain by making that the key issue of the election. In February 1856, two organizations that sided with Douglas' side of the debate had nominated separate candidates. "Douglas Democracy" nominated L. M. Keith, and a fusion organization that included "Nebraska Democrats" nominated Dyer. However, Keith refused to accept the mayoral nomination, thus Dyer was also supported by "Douglas Democracy". Dyer was nominated alongside a ticket of candidates for other municipal offices. Sherman was not. However, his candidacy was supported by the Know Nothings, who placed his name atop their own ticket. The campaign was very contentious. Individuals on either side of the debate resorted to issuing character attacks against those on the opposing side. Among incidents that arose from the political tension was a public fight between Sherman-supporting Chicago Journal editor C. L. Wilson and Dyer-supporting United States District AttorneyThomas Hoyne in late-February, which resulted in them both tumbling through a plate-glass window at the Illinois State Bank Building. Sherman was regarded to be the "anti-slavery extension" candidate, and received the backing of many Whig Party members. Dyer's campaign was considered to be well-funded. It was alleged that Douglas Democrats, in an effort to stack the vote in Dyer's favor, brought in as many as 1,500 Irish voters from neighboring Bridgeport, which was then outside the city limits, to vote illegally in the election as well as German voters. It was believed that German voters were inclined to support the "Pro Nebraska" ticket not out of their stance on the issue of the Nebraska Act but the presence of liquor candidates on the ticket. Similar allegations had previously arisen in the previous election. If that is true for the 1856 election, electoral fraud would have contributed to Dyer's margin of victory.
