Complex sentence: The Republican Party nominated John C. Frémont of California as its first presidential candidate, beating out Senator William H. Seward, and the Know-Nothing Party nominated the former President Millard Fillmore of New York.
Split into simpler sentences: The Republican Party nominated John C. Frémont of California as its first presidential candidate, who beat out Senator William H. Seward. The Know-Nothing Party nominated former President Millard Fillmore, of New York.