Complex sentence: The Opposition Party in the South was made up of former Whigs, Know-Nothings and American politicians who were pro-union, anti-Kansas, that is, anti-slavery expansion, sought a way to oppose Democrats in their states and in Congressional races.
Split into simpler sentences: The Opposition Party in the South was made up of former Whigs, Know-Nothings and American politicians who were pro-union, anti-Kansas, that is, anti-slavery expansion. They sought a way to oppose Democrats in their states and in Congressional races.