Complex sentence: New Zealand First's involvement in such a coalition would have run counter to Peters' promise to deal with the biggest party, and Turia and Sharples would have had difficulty in justifying supporting National after their supporters' overwhelming support for Labour in the party vote.
Split into simpler sentences: New Zealand First's involvement in such a coalition would have run counter to Peters' promise to deal with the biggest party. And Turia and Sharples would have had difficulty in justifying supporting National after their supporters' overwhelming support for Labour in the party vote.