Complex sentence: During the 1856 presidential election, a political smear campaign existed alleging that John C. Fremont was a Catholic in order to undermine support from those who were suspicious of Catholics.
Split into simpler sentences: During the 1856 presidential election, John C. Fremont was a target of a smear campaign alleging that he was a Catholic. The campaign was designed to undermine support for Fremont from those who were suspicious of Catholics.