Two points to make.
First, Republican Party leaders nominated Lincoln as a moderate alternative to William H. Seward of New York. The popular favorite in the Republican Party, Seward was best known for his remarks on the "irrepressible conflict." Party leaders eventually agreed that even if elected, Seward would never be accepted by Southerners.
Second, prior to October 1859, Southerners who spoke of secession as an viable political option were generally held in disfavor. Even during the campaign of 1860, no Southern Rights candidate could allow himself to be called a secessionist and be taken seriously by voters.
The "terrible mistake" interpretation of the American Civil War was first exposited by James G. Randall, "The Blundering Generation," Mississippi Valley Historical Review (June 1940), 3-28.
Michael F. Holt is among recent advocate of this position --