More than 100 million voters from across swathes of central and southern India took part in the second of five stages of the month-long vote Thursday, with experts predicting the overall result could produce a weak coalition.

Three party allies of the government say that a non-Congress politician may be needed as prime minister to appease the communists, whose parliamentary support may prove vital but who have refused to back any coalition led by the ruling party.

The wrangling points to the difficulties of political alliances building in India where smaller caste and region based parties will almost certainly hold the balance of power and influence policy making in any new government.

The Congress-led United Progressive Alliance (UPA) came to power in 2004 with communist support but the two fell out after differences over a nuclear energy deal with the United States last year.

Top communist leader Prakash Karat has ruled out support to any future Congress-led government, calling the nuclear deal an act of betrayal by the party, and relations have been reported as strained between him and Prime Minister Manmohan Singh.

The communists blocked many economic reforms, including privatizations, while it supported the government for four years.

But key Congress allies such as the Rashtriya Janata Dal (RJD) and Lok Janshakti Party (LJP) and the Nationalist Congress Party (NCP) say the support of the communists would be needed.

"We have kept our doors and windows open for the left," RJD chief Lalu Prasad told reporters late Wednesday.

The RJD and LJP, while allies of Congress, are seen as open to lending support to whichever political formulation needs them to govern.

LEADERSHIP WRANGLE

Opinion polls put the Congress-led coalition slightly ahead of a group led by the Hindu-nationalist Bharatiya Janata Party. But it could still need the support of smaller regional parties and the communists, who formed the third largest grouping in parliament after the 2004 elections.

The NCP has also said the issue of prime minister would have to be discussed after the election.

"The Congress has every right to declare Singh as prime minister. But when you take a decision on behalf of the UPA, you will have to sit together and decide," NCP chief Sharad Pawar was quoted as saying by the Asian Age newspaper on Friday.

Pawar is seen as acceptable for the top job by many of UPA allies and even the communists.

At least 55 percent of over 200 million eligible voters cast their ballots Thursday, election officials said, compared with 62 percent in the first phase last week.

The second round of polling, the biggest of the five phases, involved people from India's rural heartland, the IT center of Bangalore and some states where Maoist rebels are strong.

The staggered voting is to allow security forces to move around the country to curb any attempt to coerce an electorate more than twice the population of the United States.

The outcome of the election will be known on May 16. India's elections are notoriously hard to predict and polls have been wrong in the past. Exit polls are banned for the election.