"We will win the most seats in Bengal in 2019," Mr Shah announced today. Ms Banerjee, who was in north Bengal, said after his comments, "He said our government is a flop? ...No, he is a flop show."
"If they try to sink their teeth into Bengal, their teeth with break," she added.
This fight has spawned another one on the sidelines. Who is Bengal's main opposition party?
"Earlier Mamata Didi would only attack the CPM and the Congress. Now she is only attacking us. So the message is clear. The CPM should be worried," Mr Shah said.
The by-poll result at South Kanthi earlier this month has made the BJP upbeat. It was second with 31 per cent votes, up from 9 per cent last year. The CPM says the BJP was nowhere in the many cooperative polls the Left won recently - at Haldia, Purulia, Manbazar and Calcutta University.
"The by-election is not a reflection of the popular mood. Of course, BJP is growing but not despite Mamata Banerjee but because of Mamata Banerjee," said Md Salim, CPM lawmaker and Politburo member. "We have won many minor elections. BJP was nowhere close," he added.
But at South Kanthi, the rise in BJP's vote share equalled the fall in CPM's, something Ms Banerjee noticed. At her organizational meeting last week, she said, "The CPM vote is going to the BJP. If there are good CPM people, get them to our party. Stop them from going to BJP."
Next year's panchayat polls will fix who the main opposition party is. But 2019? Amit Shah has set that as the BJP's deadline in Bengal. Trinamool leader Partha Chatterjee laughed and said, "2019? Or does he mean 3019?"