TEHRAN—Iran’s conservatives received a landslide in the country’s parliamentary elections, strengthening difficult-liners opposed to diplomacy with the West. But a record-low turnout dealt a public rebuke to the establishment’s simply call for unity in the facial area of escalating tensions with the U.S.

Acknowledging the defeat of nonconservative factions, the reasonable President Hassan Rouhani on Sunday identified as on the new lawmakers to symbolize not just these who voted but all Iranians.