POLITICIANS other than the officially anointed successors of Deng Xiaoping are flexing their muscles in the run-up to the departure of the patriarch. Chinese sources said heavyweight cadres apparently sidelined by Mr Deng late last year, including former president Yang Shangkun and National People's Congress (NPC) chairman Qiao Shi, had grabbed the limelight in the past two weeks. They said Mr Yang, Mr Qiao and such other ''non-mainstream politicians'' as deputy NPC chief Tian Jiyun and the head of the Chinese People's Political Consultative Conference (CPPCC), Li Ruihuan, would pose severe competition to Mr Deng's three major heirs apparent, President Jiang Zemin, Premier Li Peng, and Executive Vice-Premier Zhu Rongji. ''For the sake of stability, Deng Xiaoping has not stopped this 'blooming and contention' among leaders from the various factions,'' a Chinese source said. ''Should one of his chosen successors, such as Jiang Zemin, falter, the patriarch could live with a post-Deng order dominated by the likes of Qiao Shi and Li Ruihuan.'' The source said Mr Deng had, to some extent, mended fences with former president Yang, who, together with his half-brother General Yang Baibing, were shoved aside before the 14th Party Congress last October. Xinhua (the New China News Agency) last night carried a long dispatch on a just-ended 18-day trip Mr Yang made in his native Sichuan province. The report said that, accompanied by the top leadership of the province, the former president made investigations into local agricultural problems. Mr Yang toured the homes and memorial halls associated with such deceased first-generation revolutionaries as Zhu De and Liu Bocheng. Analysts said the report confirmed Mr Yang's status as comparable to that of Mr Deng in making contribution to the communist revolution. Another ''non-mainstream politician'' who has been making waves is Mr Qiao, who outranks Mr Jiang in party seniority. Western diplomats said Mr Qiao had taken every opportunity to augment the NPC's power through pushing the principle of ''the supervision of the party and government by the legislature''. Moreover, in view of his recent trips abroad and his frequent meetings with visiting dignitaries, Mr Qiao has taken over substantial diplomatic functions. In the past week, for example, Mr Qiao, who is also a member of the Politburo Standing Committee, held meetings with politicians from both Western and Third World countries. At the same time, Mr Tian and Mr Li Ruihuan have also given speeches, or taken part in public functions, in areas beyond their formal jurisdictions. ''There is a possibility after Deng's death, a 'third force' might coalesce around relatively liberal cadres, including Yang Shangkun, Qiao Shi, Li Ruihuan, Tian Jiyun and [ousted party chief] Zhao Ziyang,'' a Western diplomat said.