BEIJING — In a year of scandals and corruption charges at the commanding heights of the Communist Party, a retired party chief some had written off as a spent force has thrust himself back into China’s most important political decisions and emerged as a dominant figure shaping the future leadership.

The resurgence of Jiang Zemin, the 86-year-old former leader, is all the more striking because he was said last year to be near death. But over recent months, Mr. Jiang, who left office a decade ago, has worked assiduously behind the scenes, voicing frustration with the record of his successor, Hu Jintao, and maneuvering to have his protégés dominate the party’s incoming ruling group. He even weighed in on how to deal with Bo Xilai, the populist political figure who was caught up in a major scandal and was investigated after his wife was accused of murdering a British businessman.

Mr. Jiang has also sought to shape policy, party insiders say, by proposing changes to an agenda-setting report presented Thursday at the start of the 18th Party Congress, the weeklong meeting that precedes the naming of Mr. Hu’s replacement and a new generation of leaders. Mr. Jiang and Mr. Hu arrived together at the Great Hall of the People, before others in the senior leadership — another sign of Mr. Jiang’s influence.

Mr. Jiang’s goal, those insiders say, appears to be to put China back on a path toward market-oriented economic policies that he and his allies argue stagnated under a decade of cautious leadership by Mr. Hu, a colorless party leader who favored more traditional socialist programs and allowed gargantuan state-owned companies to amass greater wealth and influence. Many see Mr. Jiang, who brought China into the World Trade Organization and rebuilt ties to the United States after a breakdown in 1989, as favoring deeper ties to the West and more opportunities for China’s private sector.