The government's communist allies, which provide the ruling coalition with a majority in parliament, could withdraw their support this week to protest a civilian nuclear deal with the United States.
The government is trying to secure its majority by wooing the regional Samajwadi Party into its ranks to replace the left.
The following are the most likely political scenarios, according to experts on India's political system and politicians.
IF THE COMMUNIST PARTIES WITHDRAW:
* The left parties dispatch a letter to India's President Pratibha Patil, announcing their decision to withdraw support for the ruling coalition, led by the centre-left Congress party.
* The president calls Prime Minister Manmohan Singh, informs him of the letter and asks him to convene parliament and carry out a vote of confidence in parliament, while continuing as a caretaker government.
* Singh could try to convince the president with letters from new allies that it still has a majority and can escape a vote of confidence -- a difficult scenario as opposition parties led by the Hindu-nationalist Bharatiya Janata Party (BJP), the country's main opposition party, will likely demand a vote of confidence.
* The president could also ask the BJP whether it can form a government. That possibility looks remote as the party does not appear to have the numbers and is keen on early elections.
* The government survives by mustering a majority 271 votes with support from other parties, mainly the regional Samajwadi Party, which has 39 members. The government continues in power and tries to push through the nuclear deal.
* The government falls after failing to win a majority, even with support from the Samajwadi Party and others like Rashtriya Lok Dal, which has three votes. Then an election would be called. The timing of the election would be determined by the independent Election Commission but would likely be late 2008.
* The Congress party survives as a minority government after the vote of confidence, with opposition parties like the communists abstaining from voting against the government for the time being and buying time to prepare for scheduled elections in 2009.
The minority government could then try to go ahead with the nuclear deal.