Eighteen months after the general election that swept him to power, anger in the countryside has already forced Modi to drop a policy to make it easier to take over farmland for industry and infrastructure - key to his economic plans.
And the 65-year-old's aura of electoral invincibility was dented when the upstart, anti-establishment Aam Aadmi Party crushed the BJP in a state election in the capital New Delhi in February.
"If Modi fails to redeem the lost ground by ensuring his party's victory in Bihar, his government's economic reform program will take a back seat. He'll simply not have the gall to carry out these measures, especially the politically sensitive ones," said Saibal Gupta, secretary of the Asian Development Research Institute, a private think tank.
An opinion poll published on Wednesday suggested the BJP was widening a narrow lead, but the prime minister is taking no chances.
Modi plans 20 rallies in the next few weeks in Bihar, and the state's hotels are packed with party workers from all over the country.
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While the prime minister takes the high ground promising jobs and growth, his party's campaign has now turned to religious polarization, a road-tested method of uniting fractious Hindu castes behind the BJP.
Sushil Kumar Modi, a senior party leader in Bihar who is not related to the prime minister, said if the BJP came to power in the state it would impose a strict ban on killing cows, considered holy by many Hindus. Emotions over the issue have been high after the lynching of a Muslim man in a neighboring state on rumors he had beef at home.
Since Modi took over as prime minister, several BJP states have tightened laws protecting cows.
Modi's main rival in the election, Bihar's current Chief Minister Nitish Kumar, was a former ally of the BJP who is respected for cleaning up the state's crime-ridden politics and building up its infrastructure.
But their personal political differences led to a break up of the alliance between the BJP and Kumar's Janata Dal-United in 2013.
Kumar, known for his secular, left-of-center politics, broke the partnership when Modi became the BJP's prime ministerial candidate because of Modi's alleged role in the 2002 Gujarat riots in which at least 1,000 people, mostly Muslims, died. Modi has denied any wrongdoing.
Kumar's party says the BJP is looking to create religious tensions in the run-up to the election in Bihar, where 17 percent of the population is Muslim, higher than the national figure of 14 percent. The theory is that such tensions and sectarian clashes lead to increased support for the BJP from the majority Hindus.
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Kumar's record of clean governance has taken a hit since he split with the BJP, not least because he has now partnered with a notorious veteran politician who has done prison time for corruption.
But for the state's religious minorities, Kumar's message is still preferable to that of Modi.
"We don't agree with the brand of politics that Modi and his party practice," said Muhammad Asgar, who lives in Dumrawah village, about 20 km (12 miles) from Banka, where Modi addressed the rally.
The BJP's message is two-fold.
"We've only one agenda for Bihar. And the agenda is development," said the party's national general secretary Bhupendra Yadav.
But Sushil Kumar Modi, the senior leader in the party's Bihar unit, said: "This election is a fight between those who eat beef and those want an effective ban on cow slaughter."