Complex sentence: The campaign was relatively sedate, because both the Whigs and Democrats did not organize large-scale events, possibly out of concern that rallies would be poorly attended due to defections to the Know Nothings.
Split into simpler sentences: The campaign was relatively sedate, because both the Whigs and Democrats did not organize large-scale events. THe larger parties may have been concerned that rallies would be poorly attended due to defections to the Know Nothings.