Complex sentence: Five opposition parties had earlier ruled out contesting in the by-election: the Democratic Progressive Party, the People's Power Party, the Singapore Democratic Alliance, the Singaporeans First party, and the largest opposition party in Singapore, the Workers' Party.
Split into simpler sentences: Five opposition parties, including the PPP, had earlier ruled out contesting in the by-election. The rest were the Democratic Progressive Party, the Singapore Democratic Alliance, the Singaporeans First party, and the largest opposition party in Singapore, the Workers' Party.