Final answer:
The major-party face-offs before the Civil War included the presidential election of 1860 with a split Democratic Party, and the formative Lincoln-Douglas debates of 1858, both of which revolved around the divisive issue of slavery.
Step-by-step explanation:
Two major-party face-offs that developed in the years leading up to the Civil War were primarily shaped by the contentious issue of slavery. The most significant of these was the presidential election of 1860, which saw the Democratic Party fracture into multiple factions, with Stephen Douglas representing northern Democrats, John Breckenridge for southern Democrats, and John Bell as the candidate for the Constitutional Union Party in the South.
Earlier in 1858, the Lincoln-Douglas debates showcased the rising tension over slavery, pitting the incumbent Democratic Senator Stephen Douglas against Republican challenger Abraham Lincoln. These debates helped position Lincoln as a strong candidate for the Republican Party, eventually leading to his presidential nomination and election in 1860.
However, Lincoln's election would prompt the secession of South Carolina and other southern states, signifying the imminent approach of the Civil War. Differences over the extension of slavery into the new territories and states had deeply divided the nation, leading to a tumultuous and potentially irreversible political and social rift.