Complex sentence: Since the 1980s, however, historians (notably Dorothy Thompson) have emphasised both the indispensable contribution O'Connor made to Chartism and that the causes of the movement's decline are too complex to be blamed on one man.
Split into simpler sentences: Since the 1980s, however, historians (notably Dorothy Thompson) have emphasised the indispensable contribution O'Connor made to Chartism. Further, she argues that the causes of the movement's decline are too complex to be blamed on one man.