Here's a look at the stakes at hand
The BJP hopes today to make gains in two big states - Tamil Nadu and West Bengal - that could make it less dependent on the women chief ministers who govern them, and who are often seen as fickle coalition partners.
A little over 18 crore people are registered to vote today in cities that include Chennai and Mumbai.
117 parliamentary seats will be decided today.
All 39 parliamentary seats in Tamil Nadu will be decided today. Chief Minister J Jayalalithaa's AIADMK is forecast by opinion polls to significantly improve upon the nine seats it currently holds.
"This election we are expecting a significant contribution from the south," said former BJP president Venkaiah Naidu, adding that the party wanted to lift its southern tally to 50 seats from 19 in the last election in 2009.
In West Bengal, chief minister Mamata Banerjee's Trinamool Congress is expected to win the most seats - she currently holds 19 of Bengal's 42 seats.
"We are trying to get a majority of our own. Otherwise, we will have to take the support of the regional parties and the day-to-day running of government will not be trouble-free," said BJP general secretary JP Nadda.
But earlier this week, the party's prime ministerial candidate, Narendra Modi, said he is sure that Ms Banerjee will not join his coalition after the results are counted on May 16. "We are sure that Mamataji won't join us (NDA), we have no confusion about it nor have we any apprehension about it," Mr Modi had said.
Among the heavyweights whose future will be determined today are Sushma Swaraj, the senior BJP leader who is running from Vidisha in Madhya Pradesh, former union ministers A Raja and Dayanidhi Maran who are contesting elections in Tamil Nadu, and Dimple Yadav, whose husband Akhilesh is the Chief Minister of Uttar Pradesh.
Mr Modi is filing his nomination today in Varanasi where thousands of supporters poured into the streets to cheer an open truck that he used for a massive road-show. He is also running for Parliament from Vadodara in Gujarat.
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