Complex sentence: Another prominent speaker was a Whig state assemblyman from Illinois and candidate for presidential elector from that state, Abraham Lincoln, who spoke to a crowd of 15,000 in Springfield, Illinois, wearing jeans.
Split into simpler sentences: Another prominent speaker was a Whig state assemblyman from Illinois and candidate for presidential elector from that state, Abraham Lincoln. The future president spoke to a crowd of 15,000 in Springfield, Illinois, wearing jeans.