NEW DELHI (Reuters) - Indian opposition parties have joined forces to snatch power from the country’s ruling party in a big southern state, laying the stage for other such alliances in a direct challenge to Prime Minister Narendra Modi’s re-election bid next year.

FILE PHOTO: Rahul Gandhi, President of India's main opposition Congress party, addresses his supporters during a rally described as Jan Aakrosh or public anger at Ramlila ground in New Delhi, India, April 29, 2018. REUTERS/Altaf Hussain/File Photo

A coalition of Congress and a regional group said on Sunday they will establish a government in Karnataka state next week, after Modi’s Bharatiya Janata Party (BJP) failed to prove its majority despite bagging more seats than any other party in a closely-fought election.Rahul Gandhi, the leader of the Congress party - which has struggled to make any major political inroads since Modi stormed to power four years ago - said his party will rally regional groups into a common front against Modi.

“I am very proud that the opposition has stood together and defeated the BJP, and we will continue to do so,” said Gandhi.

Karnataka, with a population of 66 million, was the first major state this year to elect an assembly, and will be followed by three more before the general election in 2019.

Political strategists say polls in Karnataka, home to India’s “Silicon Valley” Bengaluru, which was previously known as Bangalore, were seen as a key test of Modi’s popularity but the final outcome highlights the threats he faces from a united opposition are much bigger than anticipated.

“Formation of this coalition is a platform for an anti-BJP alliance for the next year,” said Sandeep Shastri, a political scientist at Bengaluru’s Jain University.

“Any shortfall in other states will further consolidate anti-BJP forces.”

Karnataka’s state’s governor last week allowed Modi’s party to form a government, even as it became clear that with only 104 seats the Hindu nationalist BJP trailed the opposition alliance, which has at least 115 seats in the 225-member assembly. That decision prompted Modi’s rivals to turn to the Supreme Court.

The governor gave the BJP 15 days to prove its majority, but the court ordered a vote of confidence in the assembly on Saturday. Even before that could take place, BJP’s newly appointed state chief minister, B.S. Yeddyurappa, resigned.

To bring the regional party - Janata Dal (Secular) - into the alliance, Congress, which has 78 of the seats, did have to concede the chief minister’s job to the smaller group. Previously, the state had been held by Congress.

“VOTE OF OVERCONFIDENCE”

Mamata Banerjee, a powerful politician in eastern India, described Modi’s failure in Karnataka as a “victory of the regional front”.

In an apparent show of strength against Modi, most opposition leaders have been invited for the upcoming swearing-in ceremony of Karnataka’s new chief minister, said Sanjay Jha, Congress’ national spokesman.

Jha said Congress’ spirit ahead of the 2019 polls was that of “necessary political accommodation” when it comes to forming alliances to stop Modi.

BJP leader Seshadri Chari said no opposition alliance will be able to stop Modi. “BJP will emerge as the single largest party (in 2019) with a majority”.

Modi remains by far the most popular politician in India and his approval rankings far outweigh Gandhi, who is the fifth-generation scion of the Nehru-Gandhi dynasty.

U.S.-based research agency Pew released a survey in November that showed nearly nine out of 10 Indians held a favourable opinion of Modi.

On Sunday, Indian newspapers carried front-page headlines highlighting Modi’s loss, a rare sight of late in Indian politics: The BJP and its allies rule 21 of India’s 29 states currently, up from seven they ruled in 2014.

“BJP loses vote of overconfidence,” said the Indian Express newspaper’s front page headline.