The law sparked lethal riots when it was passed. Now, after a four-year delay, it has come into force on the eve of Prime Minister Narendra Modi’s re-election campaign.

The incendiary law grants Indian citizenship to persecuted Hindus, Sikhs, Buddhists, Jains, Parsees and Christians from a few nearby countries. Muslims are pointedly excluded.

With a characteristic thunderclap, the government of India’s prime minister, Narendra Modi, made a short declaration on Monday night that it had finalized the details that would bring the law, known as the Citizenship Amendment Act, into force.

The government’s action, coming just before India announces the dates for an election expected in April and May, shows Mr. Modi delivering on a promise, and could change the electoral math in districts with Hindu refugees who stand to benefit from the law.

Politics aside, the law is not expected to significantly change the demography of India’s diverse population of 1.4 billion, at least not on its own. But it makes plain the power that Mr. Modi wields to redefine the Indian republic, steamrolling any resistance to his vision of a Hindu-first state.

The law spent more than four years in hibernation after protests by hundreds of thousands of Muslims and other Indians who were outraged by the idea that citizenship would be defined with reference to religious identity.

In February 2020, while President Donald J. Trump was on a state visit, riots broke out in the capital, New Delhi. Whole neighborhoods were devastated in the northeastern part of the city, where gas cylinders were turned into makeshift bombs and tossed into mosques. At least 50 people were killed, most of them Muslims.

A high-profile protest camp at a place called Shaheen Bagh, operated mainly by female protesters from different religious groups, carried on until late March before being dispersed. And then Covid-19 intervened, helping to suppress further protest.

The government justified the new rules as a humanitarian response to the plight of minorities in Pakistan, Bangladesh and Afghanistan, India’s three big Muslim-majority neighbors. Yogi Adityanath, a Hindu monk turned political ally of Mr. Modi, wrote on social media that rescuing communities “who are suffering from religious brutality” would bring “joy to humanity.”

It is hard for many to take this explanation at face value. For one thing, the inclusion of some countries and exclusion of others looks arbitrary. For another, Muslims persecuted because of their faith, for instance the Ahmadiyya and Shiites of Pakistan, do not make the cut for Indian citizenship. The Office of the U.N. High Commissioner for Human Rights called the law “fundamentally discriminatory.”

To critics, the Citizenship Amendment Act looks like one part of a pincer movement against Muslims. It was brought to life at the same time as a national citizenship registry that would allow the government to expel undocumented residents, even if their families had lived in India for generations.

As Mr. Modi’s right-hand man, Amit Shah, said at the time, “Please understand the ‘chronology’: first the C.A.A.,” and then the registry. In other words, first non-Muslim refugees would be allowed citizenship. Then the refugees who remained would be expelled. More than 1,000 “declared foreigners” have been detained in the northeastern state of Assam.

On Monday, protests erupted there and in several other states after the government announced enforcement of the citizenship law. Shaheen Ahmed, a doctoral student in Kerala, said that he and other students came out to protest across his state.

“We were demanding the rollback of the law when police came and started beating us,” Mr. Ahmed said.

One group that rejoiced at the news is a large community of lower-caste Hindus in West Bengal, whose ancestors came to India from Bangladesh. Their support for Mr. Modi in the upcoming election could tip several parliamentary seats into the majority that he is expected to achieve anyway.

Other Hindu refugees, from Pakistan, had already been acquiring citizenship. More than 1,100 have been granted that status in Mr. Modi’s home state, Gujarat, since 2016. The point of the Citizenship Amendment Act will be to make these naturalizations possible on a national level, and more visible.