BEIJING—Ahead of a top-level Communist Party conclave, Chinese President Xi Jinping is sending an unmistakable signal about what he expects from the tens of millions in the party’s ranks: total loyalty.

Culminating a weekslong state-media blitz hailing the sacrifice of Communist forces that trekked thousands of miles in the mid-1930s to find a haven to continue their revolution, Mr. Xi on Friday called for an equal display of commitment. “In our Long March of today, we must strengthen the party’s leadership, persist with strict...