A draft version of the register, published in December, excluded 4 million residents – mostly Bengali-speaking Hindus and Muslims.

In keeping with an order of the Supreme Court, the final version of the list will have to be published by July 31, amid fears the process is being rushed and many genuine Indian citizens of Bengali origin may be left out of the register on suspicion of being illegal migrants from Bangladesh.

“The NRC has become a National Register of Cruelty, nearly 4 million Bengali Hindus and Muslims have been left out and 40 of them have committed suicide over fears of loosing citizenship,” said Oliullah Laskar, a Bengali Muslim from Assam.

But such allegations have done little to faze the BJP, with its national president Amit Shah promising to extend the NRC to West Bengal and other Indian states bordering Bangladesh before rolling it out nationwide.

“If we come back to power, we will have NRC in West Bengal and the rest of India. We are committed to identify all illegal migrants from Bangladesh and drive them out,” Amit Shah said at an election rally in West Bengal, even going so far as to describe such illegal migrants as “termites”.

Deportation and detention: 3 million face statelessness in Assam

This bill was cleared by the lower chamber of parliament after it was tabled by the BJP government but failed in the upper house, where the ruling party lacks a majority. Modi has since vowed to reintroduce the bill if his party is returned to power.

Opposition parties accused the BJP of using the migration issue to heighten religious polarisation and exploit the Hindu-Muslim divide.

“The BJP’s divisive politics has led to serious social tensions,” said Bobbeeta Sarmah, who is running as a candidate for the Congress party in Assam’s capital of Guwahati.

“How can India afford a religion-driven citizenship law which contradicts its secular constitution? The BJP is attacking the very idea of India as a secular republic.”

Modi’s party has also made an issue out of two Bangladeshi actors, one of whom had overstayed his visa, joining political rallies for the Trinamool Congress in West Bengal.

Indian or not? Muslims fear being declared stateless amid drive against illegal immigrants

Both Feroze Ahmed and Abdus Nur – cast members in a number of films and soap operas in Kolkata's vibrant Bengali film industry – have been deported.

“The ease with which Nur was overstaying his visa proves how easy it is for Bangladeshis to come here and stay on illegally,” said Dilip Ghosh, president of the BJP in West Bengal, adding “this must end”.

Yet the BJP’s rhetoric has inflamed public opinion in Bangladesh, which has one of the fastest-growing economies in Asia.

“How can Indian politicians call our people termites and then expect we will be friends?” said Mohammed Hanif, a garment factory owner, who visits India regularly. “Turn on the Indian TV channels and you find some Indian politicians spewing venom against my country.”

Bangladesh’s ruling Awami League is seen as pro-India but many of its politicians are worried the BJP’s anti-Bangladesh rhetoric will damage bilateral relations.

“Such religion-driven politics in India, harping on anti-Muslim themes, will only boost Islamist hardliners in Bangladesh, who our government has tried to curb,” said Tureen Afroz, a prosecutor in the war crimes trials directed against pro-Pakistan elements in the wake of Bangladesh’s War of Independence in 1971.

In a recent article, Afroz has described some of the BJP’s statements as “evidence of incitement to genocide”.

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Bangladesh is also worried about the reaction from those of Bengali origin in Assam identified by the NRC as illegal migrants.

“Prime Minister Modi has assured our Prime Minister Sheikh Hasina that the NRC is an internal exercise and nobody will be pushed back into Bangladesh,” said a senior Bangladesh diplomat, speaking on condition of anonymity. “But BJP politicians like Amit Shah constantly talk of pushing back the illegal migrants. I am sure they will not be thrown into the sea.

“I only hope this does not become a fresh issue of friction between us at a time when India-Bangladesh relations are at their best.”

Sukhoranjan Dasgupta, a seasoned Bangladesh watcher, said the BJP’s identity politics may ultimately cost India an important ally – especially as opponents of Hasina’s government already “paint her as an Indian stooge”.

“The BJP tends to see Pakistan and Bangladesh in the same light, just because they are Muslim-majority countries,” he said.

“But Bangladesh was created after the Bengali Muslims revolted against Pakistan and won their freedom with Indian support. It is important to understand these dynamics before outright vilification of a friendly neighbour.”