Complex sentence: Although the South was friendly towards Fillmore, they feared a Frémont victory would lead to secession, and many sympathetic to Fillmore moved into the Buchanan camp lest the anti-Frémont vote be split, which might elect the Republican.
Split into simpler sentences: Although the South was friendly towards Fillmore, they feared a Frémont victory would lead to secession. Thus, many supporters moved into the Buchanan camp lest the anti-Frémont vote be split, which might elect the Republican.