An opposition bid to jointly mount a challenge to Indian Prime Minister Narendra Modi in a national election is floundering after a key party switched sides and two others broke ranks.

The developments are a boost to Modi's goal of winning nearly three-quarters of seats in the election, which is due by May.

The return of his Bharatiya Janata Party (BJP) to power in the eastern state of Bihar on Sunday after a regional party left the opposition bloc means it will contest the election running three of India's four most politically important states.

Bihar sends the fourth-most directly elected lawmakers to parliament, while its neighbour Uttar Pradesh, also ruled by the BJP, is at the top, making them critical for the ruling National Democratic Alliance's (NDA) goal of securing 400 of the 543 directly elected seats to the lower house of parliament, up from 339 now.

"There's hardly any contest left for the 2024 election now," said political analyst Sanjay Kumar.

He questioned if the main opposition Congress party would even be able hold on to its low numbers from the last two elections.

Unemployment and underemployment are a key concern for Modi's government to secure the vote, despite economic growth of 7.3%.

The BJP currently has 290 seats in the lower house compared with 48 for Congress and 16 for Bihar's Janata Dal (United), which is now with Modi's alliance partner.

Kumar's exit from the 28-party INDIA opposition alliance, formed last summer, is a blow for the grouping both numerically and optically as he was one of its main architects of the coalition. Kumar on Sunday told reporters that he left the alliance because "things were not going right".

But Congress spokesperson Pawan Khera said their campaign was intact.

"Fractures and rifts are bound to happen in the early stages of any coalition," Khera said. "Kumar's exit does not limit the INDIA alliance's political commitment."

Two other members of the alliance, the All India Trinamool Congress that rules West Bengal in the east and Aam Aadmi Party that governs Delhi and Punjab in the north, have said in recent days they plan to contest the next elections largely on their own in some key states.

Two Congress lawmakers, who declined to be named as they were not authorised to talk to the media, said Kumar's departure had forced their alliance to seek some other regional parties to join them while trying to accommodate the ambitions of existing partners. They did not identify the parties.

In a bid to garner support for the alliance, former Congress chief Rahul Gandhi is now in the middle of a 6,700 km (4,163 miles) march across the country, his second in the past year, and is slated to hold a public rally in Bihar on Tuesday.

"The opposition has to create a new game plan because the BJP has exposed how weak they are," said Sunil Ojha, a senior BJP leader in Bihar. (Reporting by Rupam Jain; Editing by Angus MacSwan)