Complex sentence: Since the early 1990s the Conservative party have had a small minority of councillors but been without wards in the constituency; a plurality of the voters in each ward have been in favour of the Labour Party and/or the Liberal Democrats.
Split into simpler sentences: Since the early 1990s the Conservative party have been without wards in the constituency. In other words, a plurality of the voters in each ward have been in favour of the Labour Party and the Liberal Democrats.