The BJP is hoping for a decisive election victory in Bihar to reaffirm Mr Modi's popularity and reinvigorate the government, after a difficult year when many crucial reforms have stalled. But high onion prices could sour the public mood in a state that grows few of its own and depends mostly on supplies from other parts of the country.
Onion prices have a politically potent history in India: they were considered a decisive factor in the outcome of two state elections in 1998, and were blamed for the fall of the central government in 1980.
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Mr Modi's government has blamed the current price spurt on unseasonal rains during the crucial March to June growing season, which yields about 60 per cent of India's total annual onion production — and provides supplies that are supposed to last consumers until October or November.
But critics believe a powerful clutch of traders who control much of India's onion supplies are working together to manipulate the market and drive up prices, as a 2012 study sponsored by the Competition Commission of India found they had done on occasion in the past.
"The root cause of the surge in onion prices is collusion among a few traders," Anil Padmanabhan, deputy editor of the Mint newspaper, wrote in a column, noting that a previous onion price spike in 2011 was contained when income-tax officials raided several large traders, and prices promptly fell 60 per cent.
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New Delhi is trying to ease supply constraints. On Sunday it raised the minimum export price to $700 per tonne, up from $425 in June, in a bid to discourage exports. It is also now trying to import 10,000 tonnes of onions from neighbouring countries. Ram Vilas Paswan, the consumer affairs minister, has also ordered a crackdown on hoarders, especially in Maharashtra.
In the meantime, the onion price spike has led to some unusual law-and-order situations. Last week a Mumbai vegetable vendor reported that thieves had stolen 14 gunny sacks of onions, weighing about 700kg, from his cart. On Sunday an onion trader reported the theft of 2,000kg of the vegetable from his warehouse in Maharashtra.
Some state governments, including Andhra Pradesh and West Bengal, have begun selling highly-subsidised onions through government stalls, but have put quotas on how much individuals can buy.