PATNA: Indian Prime Minister Narendra Modi faces a crucial test Monday when Bihar, one of India's largest and poorest states, begins voting in polls that could have major consequences for his troubled reform drive.

Modi has mounted a no-holds barred campaign, promising Biharis billions of dollars for development in a state where many of its 104 million people still vote along caste lines.

He is up against an unlikely alliance of two powerful local leaders, Chief Minister Nitish Kumar and his predecessor Lalu Prasad Yadav, who has served time in prison for corruption.

Their rivalry goes back decades, but both men — who command widespread support among the lower castes — have put their differences aside to thwart Modi, highlighting the premier's polarising nature.

Voting begins on October 12 and runs in five phases, with the results due on November 8.

Modi himself has been at the forefront of his party's campaign, addressing a host of rallies, including one on Friday near the town of Aurangabad attended by about 10,000 people.

"This 21st century election will show where Bihar stands not only on the map of India, but also on the map of the world," Modi told the crowds.

He accused the opposition of failing to better the state's fortunes in their combined six decades in power, citing high youth employment and poor power infrastructure.

Two-thirds of Biharis lack access to electricity, according to the World Bank.

"I'm supporting Modi because he wants to develop Bihar," said Sonu Jaiswal, 37, as she watched Modi in a giant field.

"We're 100 per cent sure the Modi government will win." But analysts say the outcome is too close to call.

And as criticism mounts that Modi's pledge to transform the economy is running out of steam, observers say a defeat for his right-wing Bharatiya Janata Party (BJP) will heighten the sense of declining momentum.

'Huge rivalry'

Sanjay Kumar, of the Delhi-based Centre for the Study of Developing Societies, said there was a widespread fear within the party that defeat could signal "the beginning of an end of the BJP government," even though the next general election is not until 2019.

"There is a huge rivalry in this election between what we call the backward castes and the forward castes," Kumar said.

Assembly elections are important not only because state leaders wield significant power, but because parties gain seats in India's upper house of parliament, where the BJP lacks a majority.

In a personal humiliation for Modi, the BJP lost an election in February for the New Delhi state assembly to a fledgling anti-corruption party.

The campaign for Bihar has been dogged by religious tensions after a Muslim was lynched by a Hindu mob in the neighbouring state of Uttar Pradesh (UP) last month over unsubstantiated claims that he was eating beef.

As a row over the incident escalated, this week saw legislators from the Hindu-nationalist BJP in Kashmir punch a Muslim member for throwing a provocative "beef party."

Read: Lawmaker attacked in Kashmir assembly as beef row intensifies

After 10 days of silence on the killing, Modi on Thursday used an election rally in Bihar — which is majority-Hindu but has sizeable Muslim, Christian and Sikh minorities — to appeal for unity.

A survey in the Times of India has given a Modi-supporting alliance a four percentage point lead over Kumar but India's pollsters have a mixed record in predicting the outcome of elections.

Kumar, a long-time critic of Modi, is credited with kick-starting development and quashing corruption and is seeking a third term.

His tenure contrasts sharply with that of his predecessor Yadav, a sharp-witted former railways minister who presided over years of stagnant growth and spent time in prison for embezzlement.

Some observers say Modi has put off pushing through contentious reforms ahead of the polls for fear of losing votes, such as a land acquisition bill to make it easier for firms to buy farmland.

Modi swept to power in May last year, pledging to revive the flagging economy.

While growth is now purring along at around seven per cent, complaints have been mounting about his failure to nail down major reforms and his failure to make major inroads into poverty.

Read: In India, meat and murder threaten Modi's inclusive agenda