Sunday's defeat came as Modi is being slammed publicly for failing to tackle India's explosive spike in coronavirus infections that has left the country in deep crisis
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 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 hospital and crematoriums swamped and people dying for lack of oxygen.
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 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."
Prashant Kishor, a political strategist for 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 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.
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, Modi has been expected to win the 2024 national poll.
But images of people dying from Covid-19 in hospital parking lots and corridors because of lack of beds, hospitals themselves begging for life saving oxygen supplies and overflowing crematoriums have shaken the public mood, opinion polls show.
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% saying the government has handled the Covid-19 issue ‘very’ or ‘somewhat’ well in April 2020, this number has declined to 59% at the end of April 2021, the latest data from YouGov's Covid-19 Public Monitor showed.
Covid-19 is fanning a growing anger against the federal government, said political commentator Neerja Chowdhury.