“This concept of a core will certainly help in advancing supervision inside the party and fighting corruption,” Professor Nie said.

The Central Committee also set in motion plans for a congress in the second half of next year, which is virtually certain to confirm Mr. Xi as national leader for five additional years and appoint a new cohort of officials under him.

At the congress, many leaders must retire, giving Mr. Xi room to reshape the party’s top echelon. Under the informal retirement ceiling, five of the seven members of the Politburo Standing Committee, the party’s highest body, must step down, leaving only Mr. Xi and the premier, Li Keqiang.

Naming Mr. Xi as core leader suggested that he has gained the upper hand ahead of a potentially contentious year when he and his colleagues must decide the next leadership lineup.

In the past two leadership successions, a likely heir has emerged five or even 10 years before the incumbent’s retirement as party chief. But some political insiders and analysts have said Mr. Xi may delay choosing a successor, so that he has more time and more choices, and preserves his influence.

Mr. Xi’s elevation “shows his colleagues that he is more than willing to ignore past so-called norms, which could have big implications for appointments next fall,” when the congress is likely to meet, said Christopher K. Johnson, an expert on Chinese politics at the Center for Strategic and International Studies in Washington. It “confirms that he is no first among equals, but just first.”

But the Central Committee announcement also hinted that Mr. Xi respects the need for a leadership that brings together different backgrounds. Officials should be promoted from “across the five lakes and four oceans,” said the meeting communiqué, using an old Chinese phrase for diverse backgrounds.