THE LANDSLIDE victory of the Bharatiya Janata Party and leader Narendra Modi in India’s general election is, first and foremost, a reflection of deep popular dissatisfaction with the declining momentum of what once was known as “rising India.” Turnout in the five weeks of voting was a resounding 66 percent — or more than 500 million people — and Mr. Modi’s party became the first in 30 years to capture a parliamentary majority on its own. The incumbent Congress Party suffered a historic defeat that can only be attributed to its failure to sustain India’s economic growth or effectively combat corruption.

Mr. Modi offered a compelling alternative as a leader with a record of overseeing a decade-long boom in the state of Gujarat, primed by aggressively tackling infrastructure and energy bottlenecks, paring excessive regulation and attracting private investment. He has promised to do the same for the country at large, sketching ambitious plans for new cities linked by bullet trains. A frequent visitor to China, he clearly aspires to show that India can match Chinese dynamism.

What remains to be seen is whether Mr. Modi will be the Deng Xiaopeng of India or its Vladi­mir Putin — a leader whose economic ambitions are derailed by nationalism and authoritarian temptations. For India’s 165 million Muslims, and for much of the outside world, Mr. Modi is still notorious for his lifelong membership in a quasi-martial Hindu nationalist movement and his failure to stop anti-Muslim riots in Gujarat in 2002. Though he has recently steered away from sectarianism, Mr. Modi remains intolerant of critics, both in his own party and in the media. Some accuse him of fostering a cult of personality during the election campaign, which was focused on him more than his right-wing party.

Mr. Modi will surely find it harder to monopolize power in New Delhi, even if he tries to. India’s national media are robust and independent, as are its courts. Opposition parties in Parliament will challenge him. So will some state governments. For his part, the incoming prime minister has promised to prioritize “toilets” over “temples”; he appears to recognize the dangers of diverting from the economic agenda that produced his landslide.

The United States, which a decade ago was rapidly growing closer to India, may have difficulty influencing Mr. Modi’s course. Both the Obama administration and the George W. Bush administration shunned the Gujarat leader because of his behavior during the anti-Muslim riots, and he was denied a U.S. visa in 2005. Only when his election victory looked obvious did the U.S. ambassador to India reach out to Mr. Modi. Indians already perceived the Obama administration as neglectful of their country; President Obama will have to play catch-up if there is to be a significant U.S.-Indian partnership in the coming Modi era.