Mamata Banerjee is the first female chief minister of the eastern state of West Bengal -- home to more people than Germany -- where Modi's right-wing Bharatiya Janata Party (BJP) needs to do well.
The 64-year-old is one of Modi's most vociferous critics and has been working to produce together an anti-Modi alliance.
We can tell you one thing, that we are altogether against the BJP government to save the nation," she says.
Banerjee, whose Trinamool Congress party rules the West Bengal, has succeeded in keeping the BJP away so far despite the growing popularity of Modi
The former national railways' minister known as "didi", or big sister, enjoys a mass following not least because of her humble upbringing.
- Lower-caste champion -
Known as the "Dalit Queen", Mayawati, 63, has had a colourful career as the champion of lower caste Dalits. She operates considerable influence in 200-million-strong Uttar Pradesh, India's most populous state and the election's biggest prize.
Her Bahujan Samaj Party (BSP) was collapsed in the 2014 polls but she has stitched up an alliance with her sworn enemy, the Samajwadi Party.
Analysts blamed her 2014 rout on her liking for extravagance -- she once sent her private jet to pick up a pair of sandals according to leaked US cables -- and controversy over building concrete parks full of statues of herself.
But if her party performs well this time, she could once again play a pivotal role in coalition negotiations.
Indira reborn? -
The youngest scion of the famed Nehru-Gandhi dynasty, Priyanka Gandhi Vadra entered the family business of politics in January after years of steering clear -- and has made a splash.
And what could be a better battleground for the charismatic younger sister of PM-hopeful Rahul Gandhi than Uttar Pradesh -- the Hindu heartland where Modi has his core support base.
Priyanka, 47, has been tasked with reviving the Congress party in the state, where it was thrashed in 2014 polls -- winning just two of the 80 parliamentary seats in contrast to BJP's 71.
She seems to have taken the challenge head-on, launching her campaign in Modi's constituency of Varanasi on a motorboat that traversed the Ganges, a river revered by Hindus.
Her fans say her common touch, as well as her appearance, are reminiscent of Indira Gandhi, her grandmother who was India's first woman prime minister -- and a divisive figure assassinated in 1984.
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