Complex sentence: In the 1860 presidential election, Republicans, led by Abraham Lincoln, supported banning slavery in all the U.S. territories at the time, something which the Southern states viewed as a violation of their constitutional rights and as being part of a plan to eventually abolish slavery.
Split into simpler sentences: In the 1860 presidential election, Republicans, led by Abraham Lincoln, supported banning slavery in all the U.S. territories at the time. The Southern states viewed this as a violation of their constitutional rights and as the first step in a plan to eventually abolish slavery.