"I tell all those who support the Congress that we gave unconditional support to the Congress against communalism which was in the interest of the country," the CPI-M patriarch said in a signed written statement here.
The statement also said, "Today, when the state is in danger, I specially request them to come forward and support the Left candidates to maintain peace, discipline and development."
Accusing the Trinamool and the Maoists of being hand-in-glove in perpetrating violence to endanger democracy in the state, Basu said it was time for all to come forward and support the Left for the sake of peace and development.
In the last Lok Sabha elections, the Congress-Trinamool combine defeated the LF by winning 25 of the total 42 seats.
Trinamool's alliance member Socialist Unity Centre of India bagged one seat, while the ruling coalition parties got only 15. The Bharatiya Janata Party (BJP) also secured one seat.