Since he was elected in 2014, Prime Minister Narendra Modi of India has played a cagey game, appeasing his party’s hard-line Hindu base while promoting secular goals of development and economic growth. Despite worrying signs that he was willing to humor Hindu extremists, Mr. Modi refrained from overtly approving violence against the nation’s Muslim minority.

On Sunday, Mr. Modi revealed his hand. Emboldened by a landslide victory in recent elections in India’s largest state, Uttar Pradesh, his party named a firebrand Hindu cleric, Yogi Adityanath, as the state’s leader. The move is a shocking rebuke to religious minorities, and a sign that cold political calculations ahead of national elections in 2019 have led Mr. Modi’s Bharatiya Janata Party to believe that nothing stands in the way of realizing its long-held dream of transforming a secular republic into a Hindu state.

Mr. Adityanath has made a political career of demonizing Muslims, thundering against such imaginary plots as “love jihad”: the notion that Muslim men connive to water down the overwhelming Hindu majority by seducing Hindu women. He defended a Hindu mob that murdered a Muslim man in 2015 on the suspicion that his family was eating beef, and said Muslims who balked at performing a yoga salutation to the sun should “drown themselves in the sea.”