In Assam, a Congress government is fighting for re-election against a regional opposition party and the Hindu nationalist Bharatiya Janata Party (BJP).

Both opposition parties are highlighting the illegal immigration of Muslim migrants from neighbouring Bangladesh.

The two-phase vote in Assam will be followed by polling throughout April in Kerala, Tamil Nadu and the tiny federal enclave of Pondicherry besides West Bengal in the east of the world's largest democracy.

More than 130 million voters, more than the total registered electorate in Russia, are eligible to cast ballots.

Political analysts say the Congress and its allies must win at least one of the four larger states if the coalition is to maintain its cohesion -- and keep sometimes fractious communist parties who support it from outside off its back.

A victory in any two states would boost the alliance, which is presiding over a booming economy and India's rising stature in world affairs, they said.

"Though local issues will be at the fore, national issues like the overall economy and foreign policy will also be factors," said Kuldip Nayar, independent political commentator.

"A win in two states will be a major psychological boost."

But analysts also said that a poor showing could make the coalition's communist allies, which provide it with a parliamentary majority but have refused to join the government, more aggressive as they seek to curb cautious economic reforms being pursued by Prime Minister Manmohan Singh's administration.