NEW DELHI - An upstart political party that rules India's capital has swept an election in Punjab state, a Thursday vote-count showed, bolstering its hopes of becoming the main challenger to Prime Minister Narendra Modi's Bharatiya Janata Party (BJP).

The Aam Aadmi Party (AAP), led by Delhi Chief Minister Arvind Kejriwal, was leading in 91 out of 117 seats in the northern state and looked set to beat the state's incumbent Congress party, the BJP and a regional party by a wide margin, according to Election Commission data.

"Now there will be a national alternative to the BJP and Congress," the party's spokesman, Saurabh Bharadwaj, told Reuters.

The party - its name in Hindi means "common man" - emerged in 2012 out of an anti-corruption movement and soon went on to win power in the capital, home to 20 million people.

Its appeal stems from Kejriwal's reputation as a incorruptible leader with a track record of delivering public services, analysts say.

"He's got a super clean image and he is seen as having done a very good job in Delhi," Neelanjan Sircar, a senior fellow at the Centre for Policy Research think-tank, said of the 53-year-old former bureaucrat.

Modi and his party are looking strong in the run-up to India's next general election, due by 2024. The BJP has deep pockets, formidable electoral machinery and its Hindu nationalist agenda is a proven vote-winner.

Opposition to Modi has failed to coalesce around the Congress party, which ruled India for decades but has been unable to stem a slide in its popularity over recent years.

That raises the possibility that the AAP could grow and breathe new life into an anti-Modi bloc, said Sircar.

"Over time, it's hard not to see Kejriwal and Aam Aadmi Party emerge as a serious opposition," he said.

'DELHI MODEL'

The AAP would now focus on building support in the northern states of Haryana and Himachal Pradesh, and the western state of Gujarat, Modi's home state, to show voters the BJP was not their only choice, said Bharadwaj.

"In national politics, the BJP survives on the fact that there is no national alternative - the only alternative that is suggested is Congress, which is a weak alternative," he said.

Aman Arora, a lawmaker in Punjab who was inspired by Kejriwal to join the party in 2016, said voters in the breadbasket state of more than 27 million people had been won over by the AAP's performance in Delhi and the party had to build on that.

"Our biggest strength and driving force is the governance model of the last five years in Delhi," said Arora, pointing especially to improvements in education and health that the AAP has show-cased as the "Delhi model".

Bharadwaj acknowledged that taking on the BJP was a big challenge but he said he had faith his party could use Punjab to show the rest of India just what it could do.

"We cannot compete with the BJP in terms of resources," he said.

"But when we start performing in Punjab, there will be many reasons for people to vote for the Aam Aadmi Party."

(Reporting by Devjyot Ghoshal; Editing by Robert Birsel)