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Health issues are especially sensitive among top leaders because of the need for them to appear youthful and energetic. China‘s overwhelmingly male leaders continue to dye their hair jet black well into their 70s and beyond, which helps to ward off accusations of frailty or being out of touch.

If Mr. Xi’s absence were to linger, it might also disrupt plans for the party congress, where Mr. Xi is to succeed Mr. Hu as party leader. The dates for the congress, held once every five years, were expected to be announced following a meeting of the 25-member Politburo this month, but it may have to be delayed if Mr. Xi remains out of action.

And if he is permanently indisposed, it isn’t at all clear what would happen next, because of the opacity of the party’s functions and its failure to institutionalize the succession process.

Mr. Xi was picked five years ago to succeed Mr. Hu by an undefined formula. Before that, many observers had pegged Executive Vice Premier Li Keqiang as China‘s next president. If Mr. Xi were unable to assume power, much attention will probably turn to Mr.  Li, who is now set to take over from Premier Wen Jiabao as the party’s No. 3 ranking official, with chief responsibility for managing China‘s economy, the world’s second-largest.

Yet Minxin Pei, a China politics expert at Claremont McKenna College in California, said Mr. Li would not automatically be next in line, at least not right away. A compromise candidate might rise to the fore and Mr. Li would be forced to wait until the next congress in 2017.

“Another battle would be fought,” Mr. Pei said, “with several strong contenders.”