In Washington there is a consensus that the re-election of Iranian President Hassan Rouhani is in the best interests of the U.S. Most find the self-avowed pragmatic cleric, who championed the 2015 nuclear deal, a less menacing choice than his “hard-line” opponent Ebrahim Raisi, who is rumored to be the preferred candidate of Supreme Leader Ayatollah Ali Khamenei.

But a victory for Mr. Rouhani, who appears destined to win unless Mr. Khamenei rigs the election in Mr. Raisi’s favor, would be the worst possible outcome. Better...