Complex sentence: The first Assembly election resulted in the Welsh Labour Party gaining only nearly half of the available seats, partly due to the broadly proportional electoral system that had been chosen, and probably also due to the divisive leadership battle.
Split into simpler sentences: The first Assembly election resulted in the Welsh Labour Party gaining only nearly half of the available seats. This was partly due to the broadly proportional electoral system that had been chosen, and probably also due to the divisive leadership battle.