NEW DELHI — One of India’s feistiest opposition parties was leading in early results on Sunday from elections in the state of West Bengal, a closely watched race held during a catastrophic surge of Covid-19 infections.

Top parties had campaigned relentlessly in West Bengal, one of India’s most populous states and a stronghold of opposition to Narendra Modi, the powerful prime minister. Even with cases soaring and more and more people dying across India, Mr. Modi and other politicians held enormous rallies up and down the state, which critics said helped to spread the disease.

By early Sunday afternoon, Mr. Modi’s Bharatiya Janata Party was trailing despite its heavy investment in West Bengal, a prize it desperately wanted to win. The party looked likely to win more seats in the state assembly than it did in the last election — a sign of how dominant it has become nationwide. Nevertheless, the All India Trinamool Congress party, which holds power in the state, seemed to be safely ahead.

Mamata Banerjee, India’s only female chief minister, who has developed her own cult of personality and a reputation as a street fighter strong enough to ward off the most withering attacks from the B.J.P., as Mr. Modi’s Hindu nationalist party is commonly known.

very divisive religious and citizenship issues, will remain a stronghold of the B.J.P.

working day and night, burning thousands of bodies. In New Delhi, there is an acute shortage of medical oxygen, and dozens have died gasping for breath in their hospital beds.

On Sunday, India reported around 400,000 new infections and nearly 3,700 deaths, its highest daily toll yet. Experts say that is a vast undercount and that the real toll is far higher.

Mr. Modi was scheduled to meet with his health minister on Sunday to discuss the oxygen shortage and concerns that doctors and nurses are overwhelmed and exhausted. On Saturday, Indian officials announced that the first batch of the Russian vaccine, Sputnik V, had arrived, a boost to India’s flagging inoculation campaign.

Critics have blasted Mr. Modi’s handling of the crisis. His government failed to heed warnings from scientists, and its own Covid-19 task force did not meet for months. To signal that India was open for business, Mr. Modi himself declared a premature victory over Covid in late January, during what proved to be a mere lull in infections.