EXHILARATED by his party's showing against the ruling People's Action Party (PAP) in the 1991 general election, Chiam See Tong, secretary-general of the Singapore Democratic Party (SDP), said he did not see why the opposition could not form the next government ''in the not too distant future''. Today, Mr Chiam is no longer secretary-general of the SDP, which he helped found in 1980, and the prospect of the PAP losing power to an opposition party has never looked more remote. His resignation as secretary-general, announced by the Central Executive Committee of the SDP, was explained as being part of the party's policy to nurture a younger generation of leaders for the next century. It also praised Mr Chiam for keeping the ''candle of democratic politics alight despite the best efforts of the PAP to snuff out the flame''. Mr Chiam quickly countered the impression that all was well, saying his resignation was a deliberate move to expose divisions within the party ranks. All this added up to a severe set-back for opposition politics in Singapore, where the PAP has been in power for three decades. It came just as the opposition appeared to be taking strides forward, following years of failed efforts to make some headway against the PAP and its roughhouse tactics. On the face of it, Mr Chiam's and his colleagues' parting of the ways has cast serious doubts on the SDP's ability to become a credible alternative party. Until 1991, the opposition had never been represented by more than two people in parliament. That year, the SDP won three seats and the Workers' Party (WP) one. The PAP saw its share of valid votes fall from 63.2 per cent in 1988 to 61 per cent in the 1991 election and a downcast Prime Minister Goh Chok Tong was forced to deny rumours that he was resigning. Mr Chiam was recognised in parliament as unofficial leader of the opposition and the SDP reported a boost in membership applications. The party's prospects looked even brighter last December when Mr Chiam introduced Chee Soon Juan as the SDP candidate for a by-election called by the prime minister in his constituency of Marine Parade. An articulate, 30-year-old National University of Singapore (NUS) lecturer, Dr Chee was presented as the face of the opposition's future. Large crowds turned out to cheer him on at by-election rallies, but the party failed to translate this public support into votes. Mr Goh won with an increased margin, renewing his confidence in the PAP's ability to keep the opposition a weak force. The opposition might have done better if the SDP and the (WP) had worked together. But talks between Mr Chiam and his WP counter-part, J. B. Jeyaretnam, failed to come up with a plan for co-operation. In April, Dr Chee's credibility also suffered when he launched a hunger strike in an unsuccessful attempt to overturn his dismissal by the NUS for alleged dishonesty. Even SDP colleagues said it was a fight he could not win. As it happened, he merely provided the PAP with an opportunity to lambaste him in parliament and he ended up being hit with a number of slander suits. As the PAP takes full advantage of another opposition setback, the SDP may have to wait a little longer before it tries to prepare for power.