A Congress legislator along with his supporters joined the Trinamool Congress on Saturday.
Baduria MLA Qazi Abdur Rahim joined the ruling party in West Bengal at its headquarters in the presence of senior ministers Firhad Hakim and Partha Chatterjee.
"I have realised that Didi (Chief Minister Mamata Banerjee) is the only hope in the fight against the BJP. So, I am joining the TMC to strengthen her hand," Mr Rahim said.
Senior Congress leader Manoj Chakraborty said the induction of opposition Congress and CPI(M) MLAs into the TMC reflects the ruling party's intention to "weaken secular forces" in the state.
The Congress had won 44 seats in the 2016 assembly elections but 18 of its MLAs have switched over to the ruling TMC since then.
However, barring former Sabang legislator Manas Bhunia, who later became a Rajya Sabha MP, none of the Congress turncoats have resigned from their MLA posts.
Besides Rahim, former BJP Mahila Morcha vice president Moumita Basu Chakraborty and some retired bureaucrats also joined the TMC.
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