Complex sentence: However, historians have also come to regard Van Buren as integral to the development of the American political system, and his most lasting achievement was as a political organizer who built the modern Democratic Party and guided it to dominance in the new Second Party System.
Split into simpler sentences: However, historians have also come to regard Van Buren as integral to the development of the American political system. His most lasting achievement was as a political organizer who built the modern Democratic Party and guided it to dominance in the new Second Party System.