The chief of communist party has said they would stop the Bharatiya Janata Party from taking power, in a sign of its softening stand against the ruling Congress that may help it to form a coalition.
The communists, who supported the Congress-led alliance for more than four years before parting ways angry over a nuclear deal with Washington last year, had ruled out backing any future Congress coalition.
The Congress-led coalition is just ahead of the Bharatiya Janata Party (BJP)-led alliance after a mammoth general election but both need new allies for a parliamentary majority, according to exit polls on Wednesday.
The communists spearhead the "Third Front", a loose alliance of smaller parties, but the exit polls showed the combination faced possible disintegration.
"The Left parties and our allies in the Third Front will not give BJP an opportunity to exploit the post-poll situation to install its government," Prakash Karat, chief of the Communist Party of India (Marxist) told The Economic Times newspaper.
The left has always seen the BJP as an extremist religious group.
The communists are projected to win only about half of 59 seats they won in 2004, weakening their leverage over any post-poll bargaining.
This could mean the communists and some of the Third Front partners were keeping their option open of tactically supporting Congress in forming a government.
"The left has come to terms with reality that the Third Front is not a possibility and that is why they have changed their stance," D.H. Pai Panandikar, president of the RPG Foundation, a private think tank.
"In order to prevent the BJP, they would rather make an alliance with Congress. That is a practical position and a legitimate one," said political analyst Pran Chopra.
Political commentator Mahesh Rangarajan said Karat's comment signalled the left were keeping all options open.
"The left will first try to unify regional parties and push for an alternative government," Rangarajan said.