In his original letter to Lincoln, Hurlbut had suggested that members of the old Whig Party leaned toward supporting Lincoln’s candidacy for the U.S. Senate while the old Democratic ranks that had become Republicans leaned toward Douglas. Despite Lincoln's confidence, some Republicans were excited by Douglas’ repudiation of the Lecompton Constitution to the extent that they considered supporting his bid for reelection. Although Douglas later denied it, he courted Republican support—meeting in person with prominent men such as Horace Greeley and hinting in correspondence to Republicans that he was finished with the Democratic Party. Lincoln and other Republicans were concerned by these developments and urged fellow party members to remain loyal in the upcoming election.

Greeley and other editors of Republican newspapers in the East continued to back Douglas, but the rank and file Republicans in refused to follow suit. Republican conventions were held in one hundred counties of Illinois to choose delegates to the state convention, and ninety-five of them passed resolutions supporting Lincoln for U.S. Senate. Delegates to the state convention unanimously nominated Lincoln to run against Douglas. Lincoln’s attempt to unseat Douglas did not prove successful, but his involvement in the campaign, particularly the Lincoln-Douglas Debates, lifted Lincoln to national prominence.