Indian Prime Minister Narendra Modi is rallying his nationalist base ahead of the world's biggest election, which starts on Thursday, but has become tighter than anticipated just months ago, because of falling incomes for farmers and a lack of jobs.

Nearly 900 million people are eligible to vote in an election spread over seven phases into next month to allow security forces and officials to ensure a free and fair ballot.

Voters will travel by buses, trains, boats to about a million polling stations. Votes will be counted on May 23 and results are expected the same day.

Polls predict the alliance led by Modi's Bharatiya Janata Party (BJP) will just about squeeze out the parliamentary majority needed to rule, a sharp drop from his commanding mandate five years ago, when he vowed to turn India into an economic and military power.

Read more: Has Indian PM Modi fulfilled the economic promises he made to voters in 2014?

But his government's inability to create a million jobs every month for new entrants to the workforce, and ease farmers' distress over low product prices, has taken the shine off what is still the world's fastest growing major economy.

From sugar farmers in northern Uttar Pradesh going unpaid for their produce, to small businesses in the south shuttered because a new, unifying national tax is too challenging to comply with, discontent has brewed for months, pollsters say.

In December, alarm bells clanged within Modi's Hindu nationalist fold after he lost three big battleground states in a final test of popularity to the main opposition Congress and its allies, led by Rahul Gandhi.

Analysis: Indian state polls spell setback for Modi, boost for Congress

But a surge in tension with Pakistan in February has pushed Modi ahead as he plays up his image as a defender of national security and paints his rivals as weak-kneed, even questioning their patriotism at times.

"People were very unhappy, angry that Modi makes tall promises and doesn't deliver," said Shiv Chandra Rai, an Uber driver in the commercial capital of Mumbai, who said his unemployed brothers faced difficulties making ends meet in their home village in Uttar Pradesh.

"Everyone said there are no jobs, everywhere farmers are struggling. But on this issue of Pakistan we are confused now. Some people feel we have to vote for Modi on this issue, it is a national problem."

Even as tension between both countries subsided, Modi vowed more similar action if militant attacks continue in Indian-occupied Kashmir, and he attacked his critics for questioning the efficacy of the strikes.

"Is their hatred for Modi so great they can't even unite when it comes to national security?" he said to tens of thousands of cheering supporters wearing saffron headbands.

The Congress, led by Gandhi, and his charismatic sister Priyanka Gandhi Vadra who took up a party post in January, is trying to steer the campaign back to Modi's broken promises on the economy.

Read more: Modi or Gandhi? Indian mystics split over poll outcome

Gandhi has pledged a monthly payment of Rs6,000 to India's poorest families, benefiting 250 million of a population of 1.3 billion, in what he called a final assault on poverty.

"The BJPs campaign is largely around nationalism and national security., and this is echoed in their manifesto," said Sanjay Kumar, director of think tank the Centre for the Study of Developing Societies in New Delhi.

"Congress is trying to pitch in the election with regard to farm distress, rural crisis, unemployment."

Gandhi, as Congress leader, had attempted to forge a grand alliance with a clutch of powerful regional parties fighting the elections, so as to avoid the splintering of votes.

But the unwillingness of other Congress officials to give in to the demands of the smaller parties and the vaulting ambition of key regional figures has frustrated his efforts.

Still the Congress said Modi represents a threat to every opposition group by pursuing a vision of a Hindu-first India, stoking fear among India's Muslim population. The BJP denies such bias.

Read more: Hindu nationalism marches ahead in India's general elections

Modi's tactics were borrowed from the playbook of US President Donald Trump, said Sam Pitroda, an adviser to Gandhi.

"The strategy was, there is an enemy at the border. In that case it was Mexican, and enemy, also, as people in the country, immigrants. Very similar, Modi says there is an enemy at the border... Anything you say, immediately everyone attacks you."