Original sentence: Withdrawal has been advocated by Eurosceptics, both left-wing and right-wing, while Pro-Europeanists (or European Unionists), who also span the political spectrum, have advocated continued membership
Edited sentence: Withdrawal has been advocated by Eurosceptics, both left-wing and right-wing, while Pro-Europeanists (or European Unionists), who mainly consist of centrist liberals, have advocated continued membership