FACTBOX - Potential swing states in India elections

A Sadhu or a Hindu holyman shows his ink-marked finger after casting his ballot outside a polling station in Mathura May 7, 2009.

(Reuters) - Millions of Indians began voting in the penultimate round of a general election on Thursday, including the possible swing state of West Bengal where the ruling Congress-led alliance hopes to win crucial seats.

Here are details of five most important swing states which account 243 of 543 parliamentary seats in the April-May election. The main vote battle is between Congress and the Bharatiya Janata Party (BJP).

* UTTAR PRADESH (80 seats)

India's most populous state could be a key election with an outside chance the controversial chief minister Mayawati could become the country's next prime minister.

Mayawati's Bahujan Samaj Party (BSP), which champions the cause of Dalits or former "untouchables", will probably be challenged by Congress and the Samajwadi Party (SP), who could defeat Mayawati if they pool their vote share.

But there is a chance the current Congress-SP alliance, seen by analysts as a marriage of convenience, may crumble.

If Mayawati builds on her success at the state level she could be "kingmaker" in case of a hung parliament. She could even be prime minister as head of the "third front" -- a coalition of smaller parties -- if the two main parties falter.

Congress has in the past deployed two of its star campaigners, party chief Sonia Gandhi and her son Rahul, in the state but failed to make significant inroads.

* ANDHRA PRADESH (42 seats)

Congress won the southern state in 2004 with the help of the regional Telangana Rashtra Samiti (TRS) party, but the TRS has since walked out of Congress' coalition.

In a likely three-horse race, Congress will compete against a "grand alliance" of smaller parties including the TRS, and against the actor-turned-politician Chiranjeevi, whose recently created party has good prospects in some pockets of the state.

The BJP is unlikely to have major impact.

* TAMIL NADU (39 seats)

The southern state of Tamil Nadu is said to have tipped the balance of power in favour of Congress in 2004 after it won the support of the Dravida Munnetra Kazhagam (DMK).

The DMK won a resounding election victory but there are signs its support could be faltering. Its main rival, the All India Anna Dravida Munnetra Kazhagam (AIADMK), has already reached out to Congress for a potential post-election alliance.

But Congress is almost certainly going to stick with its current ally. The BJP still has no major ally in the state.

* BIHAR (40 seats)

The Congress-led coalition won a majority in the populous, eastern state of Bihar in 2004. But since then the Janata Dal (United) (JD(U) party, a member of the coalition, has trounced its opponents in state elections and dislodged chief minister Rabri Devi, the wife of India's railway minister, Lalu Prasad.

The JD(U) is a BJP ally but there has been speculation it could switch to Congress after the polls.

* WEST BENGAL (42 seats)

The eastern state of West Bengal has been run by the left for decades. The left was a key Congress ally after elections in 2004 but walked out of the coalition last year in protest against a civilian nuclear deal with the United States.

The left might choose to return to its former ally this year, especially if Prime Minister Manmohan Singh, who staked his political future on the deal, steps down.

But for the campaign, Congress has tied up with the state's main opposition, Mamata Banerjee of the Trinamool Congress party.

(Additional reporting by Sujoy Dhar and S. Murari)

(here)