Complex sentence: Former president Millard Fillmore's "Know-Nothing" candidacy helped Buchanan defeat John C. Frémont, the first Republican candidate for president in 1856, and he served from March 4,1857, to March 4,1861.
Split into simpler sentences: Former president Millard Fillmore's "Know-Nothing" candidacy helped Buchanan defeat John C. Frémont, the first Republican candidate for president in 1856. He served as president from March 4,1857, to March 4,1861.