Sinha, a former parliamentarian of India's ruling Bharatiya Janata Party, was inducted into the Trinamool Congress in state capital Kolkata in the presence of the regional party's top leaders, days ahead of the assembly elections.
"The tipping point was the attack on Mamata Ji in Nandigram. It was the moment of decision to join TMC and support Mamata Ji," the 83-year-old told the media, after formally joining the party.
Sinha, who also briefly served as India's foreign minister in Prime Minister Atal Behari Vajpayee's Cabinet in the late 1990s, quit the BJP over differences with the leadership in 2018. His son, however, is a BJP parliamentarian from the eastern state of Jharkhand.
Coming down heavily on BJP, the former Indian Finance Minister said, "The country is facing an unprecedented situation today. The strength of democracy lies in the strength of the institutions of democracy. All these institutions have become weak now."
A couple of days back, Sinha slammed his former party over the alleged attack on Banerjee. “Shame on BJP. Instead of sympathising with Mamata injured in an attack they are making fun of it,” he tweeted on Wednesday.
A day after, he wrote on Twitter: "The battle for Bengal is the battle for India. The voters in Bengal will vote for the future of India in this election."
On Wednesday evening, Chief Minister Banerjee claimed that she was attacked by four to five men outside a temple in the Reyapara area of Nandigram town, some 130 kms from state capital Kolkata, when there were no security personnel around her.
"It's a conspiracy, there were no policemen around me at the time," she had said, before being whisked away by her bodyguards in her official car. She was in Nandigram to file her nomination papers to fight the assembly elections, slated to begin later this month.
Nandigram is, in fact, the potboiler of assembly elections in Bengal, where Banerjee has been challenged by her erstwhile close aide Suvendu Adhikari, who has now joined BJP.
While the Trinamool Congress had condemned the attack on Banerjee, with party MP and Bengali actress Nusrat Jahan tweeting, "You may try to harm her, but our Didi shall always stand tall", the BJP had called the attack a "drama".