Complex sentence: The first African American to assume the presidency, he previously served in the United States Senate representing Illinois from 2005 to 2008 and in the Illinois State Senate from 1997 to 2004.
Split into simpler sentences: The first African American to assume the presidency, he previously served as a United States Senator, representing Illinois from 2005 to 2008. He also served in the Illinois State Senate from 1997 to 2004.