Complex sentence: Having grown up in a Whig household, Harrison initially favored that party's politics, but joined the Republican Party shortly after its formation in 1856 and campaigned on behalf of the Republican presidential candidate, John C. Frémont.
Split into simpler sentences: Having grown up in a Whig household, he favored that party's politics while young. He joined the Republican Party shortly after its formation in 1856, and that year campaigned on behalf of the Republican presidential candidate John C. Frémont.