Three people who are close to senior officials said that before Mr. Sun was dismissed, he was put under investigation by the party. Two of them said an internal party explanation of his removal did not specify allegations but said investigators had found “violations of discipline,” a vague term that might mean political infractions or corruption. The three spoke on condition of anonymity, citing fear of punishment for discussing internal party affairs. The Wall Street Journal first reported the inquiry.

News outlets in Chongqing were told to stop using Mr. Sun’s keynote slogans for developing the area, according to an internal memo an editor at an official newspaper sent to his staff.

Mr. Sun’s removal appears to undermine the pecking order of elite promotion that had been taking root under Mr. Xi’s predecessors, especially Hu Jintao, the previous president. Mr. Sun’s promotion was never fail-safe, but under the hierarchy created at Mr. Hu’s retirement, he had appeared poised for elevation this year.

“If Sun Zhengcai is not promoted and in fact being brought down, being purged,” said Susan L. Shirk, the chair of the 21st Century China Center at the University of California, San Diego, “that really is an indication that the unwritten rules, or norms, of leadership succession are not being followed.”

She saw the move as part of an effort by Mr. Xi to “consolidate as much power as he can” without being challenged by other members of the party elite.

“There have been people purged before and corruption has been the excuse, too,” she said. “But it seems like there’s been more of that under Xi Jinping.”

Mr. Xi has not commented publicly on the moves, but he appears to be maneuvering to ensure that the Politburo and its all-powerful standing committee are dominated by loyalists.