Complex sentence: Both election results were bitterly contested by Democratic opponents, and the campaigns and elections had been surrounded by violence and intimidation, particularly by the paramilitary White League, whose armed members rode to turn out Republicans and suppress black voting.
Split into simpler sentences: Both election results were bitterly contested by Democratic opponents, at a time when they used violence and intimidation to disrupt campaigns and elections. The paramilitary White League had armed members who rode to turn out Republicans and suppress black voting.