Four major candidates accepted nominations for the presidency. The Democratic Party had great difficulty selecting a candidate because party members could not agree on the major issue of the day: the expansion of slavery in the Western territories. After Southern Democrats walked out of two of the party's national conventions, Northern Democrats nominated Stephen Douglas of Illinois to represent them in the presidential election. Douglas ran on a platform of popular sovereignty; he wanted the people of each territory to vote on whether or not to accept slavery in their region.
This does NOT look like 4 sentences! X3