AP Photo

China's departing president, Hu Jintao, entered the summer in an apparently strong position after the disgrace of Bo Xilai, previously a rising member of a rival political network who was brought down when his wife was accused of murdering a British businessman. But Mr. Hu suffered a debilitating reversal of his own when party elders — led by his predecessor, Jiang Zemin — confronted him with allegations that Ling Jihua, his closest protégé and political fixer, had engineered the cover-up of his son’s death. According to current and former officials, party elites, and others, the exposure helped tip the balance of difficult negotiations, hastening Mr. Hu’s decline; spurring the ascent of China’s new leader, Xi Jinping; and playing into the...