Complex sentence: The PSD contested the elections as the lead party in the of the National Front alliance, with the Tunisian General Labour Union, whilst three other parties also ran; the Movement of Socialist Democrats, the Popular Unity Movement and the Tunisian Communist Party.
Split into simpler sentences: The PSD contested the elections as the lead party of the National Front, an alliance with the Tunisian General Labour Union. Three other parties also nominated candidates--the Movement of Socialist Democrats, the Popular Unity Movement and the Tunisian Communist Party.