NEW DELHI (REUTERS) - Indian opposition parties and political commentators cheered the election victory of a regional leader over Prime Minister Narendra Modi's ruling party in a big battleground state as a sign his populist sway could be checked.

Sunday's defeat came as Mr Modi is being slammed publicly for failing to tackle India's explosive spike in coronavirus infections that has left the country in deep crisis, with hospitals and crematoriums swamped and people dying for lack of oxygen.

Mr Modi addressed dozens of political rallies in the state of West Bengal, hoping to widen the appeal of his Hindu nationalist Bharatiya Janata Party (BJP) to the east of the country from its traditional northern and western strongholds.

But West Bengal Chief Minister Mamata Banerjee, who ran the campaign for her regional party from a wheelchair because of a fall at a rally, won a two-thirds victory, raising opposition hopes that Mr Modi could be challenged across the country.

"What Bengal does today, India does tomorrow," columnist Shobhaa De wrote in The Print, paraphrasing a quotation by 19th century liberal Gopal Krishna Gokhale.

"What happened in West Bengal is just the beginning."

Mr Prashant Kishor, a political strategist for Ms Banerjee, said: "The election result has given voice and hope to those who want to fight this danger called BJP."

The Shiv Sena, another regional group that controls the western state of Maharashtra that includes Mumbai, said that the election result was a personal defeat for Mr Modi because he put everything on the line and ignored the health crisis.

"Instead of tackling the raging Covid-19 pandemic, the entire central government, including Prime Minister Narendra Modi, was in the poll arena of West Bengal to defeat (Chief Minister) Mamata Banerjee," it said.

Iron grip

Mr Modi has held an iron grip on Indian politics since sweeping to power in 2014 and winning a bigger victory in the 2019 national election on the back of a strong Hindu ideology.

Until now, there has been no challenger, and with the main opposition Congress party unable to get its act together, Mr Modi has been expected to win the 2024 national election.

But images of people dying from Covid-19 in hospital carparks and corridors because of a lack of beds, hospitals themselves begging for life-saving oxygen supplies, and overflowing crematoriums have shaken the public mood, opinion polls showed.

Confidence in the government's handling of the crisis has plummeted since February when the second wave of infections started, according to a survey among urban Indians by polling agency YouGov.

From 89 per cent saying the government has handled the Covid-19 issue "very" or "somewhat" well in April last year, this number has declined to 59 per cent at the end of April this year, the latest data from YouGov's Covid-19 Public Monitor showed.

Covid-19 is fanning growing anger against the federal government, said political commentator Neerja Chowdhury.

"People are not likely to forget the shortage of hospital beds, oxygen and vaccines in a hurry. They are also unlikely to forget in a hurry that the BJP's central leadership made winning Bengal its life-and-death battle, when there is a real life-and-death struggle in the country."