Joseph R. Biden Jr. ran for the White House as a sharp critic of President Donald J. Trump’s crackdown on trade with China. In office, though, he has taken Mr. Trump’s trade war with Beijing and escalated it, albeit with a very different aim. The two men, locked in a rematch election this fall, share a rhetorical fondness for beating up on China’s economic practices, including accusing the Chinese of cheating at global trade. They also share a building-block policy for countering...