Beijing was further encouraged by Mr. Trump’s frequent claim of friendship with Chinese President Xi Jinping and by Mr. Trump’s praise for Chinese Vice Premier Liu He for pledging to buy more U.S. soybeans.

An April 30 tweet, in which Mr. Trump coupled criticism of Mr. Powell with praise of Chinese economic policy, especially caught the eye of senior officials. “China is adding great stimulus to its economy while at the same time keeping interest rates low,” Mr. Trump tweeted. “Our Federal Reserve has incessantly lifted interest rates.”

“Why would you be constantly asking the Fed to lower rates if your economy is not turning weak,” said Mei Xinyu, an analyst at a think tank affiliated with China’s Commerce Ministry. If the U.S.’s resolve was weakening, the thinking in Beijing went, the U.S. would be more willing to cut a deal, even if Beijing hardened its positions.