“I will skin you!” she shouted in a confrontation caught on video. “Write down the names. Search the whole locality, house by house. How dare you!”

Mr. Ghosh joined Ms. Banerjee’s party soon after retiring as a civil servant in the 1990s. As a close political adviser, he was central to her 2011 victory to become chief minister in the state, with a population just short of 100 million.

In return, she had promised to nominate him for a local assembly seat. The rupture between them emerged when Ms. Banerjee announced the name of one Dipak on the candidate list, just not Dipak Ghosh.

The B.J.P. decided to contest local elections vigorously in West Bengal last year. While it did not make many inroads then, the party’s willingness to sacrifice effort, cash and even personnel — around 45 B.J.P. supporters are thought to have been killed in political violence in that season — made an impression on the state’s voters that it was here to stay, Mr. Ghosh said.

“I don’t think she realized it fully,” Mr. Ghosh said of the B.J.P. threat. “They gave blood.”

Even Ms. Banerjee’s critics characterize her as a hard-working and seemingly tireless fighter.

She is in her element on the streets and at rallies, arriving in her trademark white sari, wearing white flip-flops, her hands clasped in respect. Then she grabs the microphone, pacing the stage left and right.

“She knows how to connect with the common man,” said Debasish Bhattacharya, who runs a magazine supported by Ms. Banerjee’s party. “She has fought against the odds throughout her life. But this is a much bigger fight, and she has also become older. This will be a tougher fight.”