India's opposition faces a "herculean task" in next year's general elections against Indian Prime Minister Narendra Modi's Bharatiya Janata Party (BJP), which wrested control of key states in a surprisingly strong showing in local polls.

The defeat of Congress in all three heartland states, which was announced on Sunday, dashed any notion that the opposition could pose a serious challenge through a newly formed 28-party alliance led by the party that has ruled India for 54 years since independence from Britain, analysts and politicians said.

It also showcased the organisational strength of the BJP and the appeal of Modi's presidential-style campaigning, even though he was not on the ticket, just five months before national elections in which he will seek a third term in office.

"After these results, it looks like there is no stopping Modi," said Yashwant Deshmukh, poll expert with C-Voter agency, adding that stopping Modi would be a "herculean task". The BJP won the regional votes in three of four major states, including central Rajasthan and Chhattisgarh which were ruled by Congress.

Modi swept to power in 2014 on a pitch for stability and growth from what he described as the corruption and poor governance of Congress. He consolidated his victory by boosting the economy, offering more welfare and pushing an aggressive brand of Hindu nationalism, winning a second term in 2019.

He remains popular and surveys show he is the favourite to win the 2024 election.

Congress leader Rahul Gandhi has worked hard to revive the party in recent years. He led a 135-day march across the country, changed the party's leadership and pushed to form the 28-party alliance called INDIA, wooing voters with payouts.

A Congress victory in the big southern state of Karnataka this year was touted by the party as the beginning of its comeback - until Sunday's results came in and prompted soul searching within the party instead.

"We need to go back to the drawing board and seriously assess what really went wrong, why we were not able to get the confidence of people," senior Congress lawmaker Manish Tewari told Reuters. "You need a programme which will enthuse people, which will be seen as an alternative."

One of the challenges facing the opposition are the factious ties within the INDIA alliance.

Congress refused to share seats in state polls with key regional ally the Samajwadi Party. Spokesperson Manoj Kaka told Reuters Congress did not seem "fully committed" to the alliance.

"The future of the alliance is good if the Congress party works on it with full commitment," he said.

Asked about the fate of the alliance, K.C. Venugopal, a senior Congress leader and close aide of Gandhi, said alliance partners would hold talks soon. He declined to elaborate.

After Sunday, the BJP now foresees a "massive victory", said party Vice President Baijayant Panda.

"The severe bickering among themselves leaves zero hope for this so-called alliance to pose any challenge to the BJP whatsoever," Panda added. (Additional reporting by Saurabh Sharma in Lucknow; editing by Miral Fahmy)