HONG KONG — The start to the day was hardly unusual for a senior Chinese leader in a country grappling with an economic slowdown. On the morning of July 24, Zhou Benshun attended a meeting to promote one of President Xi Jinping’s signature projects, a plan to boost growth by building a “supercity” that would integrate Beijing with the region around it.

But by 6:10 p.m. that day, Mr. Zhou’s career was over, and he faced years in prison.

The Communist Party’s anticorruption agency announced it was investigating him on “suspicion of serious violations of party discipline and the law,” signaling his ouster as the party chief of Hebei Province, one of the nation’s most populous.

Mr. Zhou’s sudden downfall — he is the first sitting provincial party chief to be purged by Mr. Xi — underscores the uncertainty that permeates the Communist elite as they contend with two unnerving developments beyond their control: an economic slowdown that appears to be worse than officials had anticipated and that could mark the end of China’s era of fast growth, and a campaign against official corruption that has continued longer and reached higher than most had expected.

Driving decisions on both issues is Mr. Xi, who took the party’s helm nearly three years ago and has pursued an ambitious agenda fraught with political risk. Now, weeks before a summit meeting in Washington with President Obama, those risks appear to be growing, and there are signs that Mr. Xi and his strong-willed leadership style face increasingly bold resistance inside the party that could limit his ability to pursue his goals.