Indian Prime Minister Narendra Modi’s BJP party is heading for a historic victory in the north-eastern state of Assam, one of five states electing new governments.
The party is ahead in 61 of the 126 seats, while the Congress is leading in 25 seats, the Election Commission said.
Assembly elections were held in Tamil Nadu, Kerala, West Bengal and Assam and in the union territory of Pondicherry.
The main opposition Congress is poised to lose in all the states.
The BJP is also trailing in the other states.
The Assam results have come as a shot in the arm for the BJP which swept the national polls in the summer of 2014 but has since suffered setbacks in important regional elections, including in the capital, Delhi, and in the northern state of Bihar.
The party has won 43 seats and is leading in 18 others.
Assam is ethnically diverse and prone to conflicts and the state has been governed by the Congress since 2001.
For these elections, the BJP tied up with local parties that support rights of local tribespeople and are opposed to illegal migration and, correspondents say, that worked in its favour.
In the eastern state of West Bengal, the regional Trinamool Congress party led by Chief Minister Mamata Banerjee is poised for a second term in power.
In the 294-seat assembly, her party has won 183 seats and is leading in another 28, while a rival alliance of the Congress and left-wing parties has won 56 seats and is ahead in 20.
In the 234-member Tamil Nadu state assembly, the regional AIADMK has won 96 seats and secured a lead in 36 others, while its nearest rival, the DMK, has won 67 and is leading in 22 seats.
In Kerala, which has been traditionally ruled by either Congress-led governments or a coalition of left-wing parties, the Communist Party of India (Marxist) has won 57 seats and is leading in one more, while the ruling Congress has won 19 and is ahead in three seats.