Complex sentence: The Anti-Mason party, who had earlier held their convention also in Baltimore in September 1831, nominated William Wirt of Maryland and Amos Elmaker of Pennsylvania; both Jackson and Clay were masons.
Split into simpler sentences: The Anti-Mason party, had earlier held their convention in Baltimore in September 1831. They nominated William Wirt of Maryland and Amos Elmaker of Pennsylvania; both Jackson and Clay were masons.