Through her revolt within the Trinamool Congress, Kakoli Ghosh Dastidar keeps coming back to a number — she says “nearly 20 MPs” are with her, against party founder Mamata Banerjee. The figure that really matters, though, is 19. It decides whether the rebels can switch sides and still keep their Lok Sabha seats, or lose them.Kakoli Ghosh says she and others are leaving Mamata Banerjee's TMC for it has been corrupt. Raghav Chadha did something similar against Arvind Kejriwal's AAP earlier this year. (File Photos: ANI, PTI, HT)And it echoes a move made just months ago by AAP's Raghav Chadha.Here is how it works.What law says, what Chadha usedThe anti-defection law, set out in the Tenth Schedule of the Constitution, has the aim to stop elected members from jumping parties for power or money, or mainly switch loyalties when voters have elected them on a certain party's symbol. The basic rule is that if you are elected on a party's symbol and then leave that party or vote against its instructions ( a ‘whip’, as it's called in parliamentary lexicon), you can be disqualified.If at least two-thirds of a party's legislators agree to merge with another party, they are protected, says a provision. There used to be an easier escape hatch for such a "split", that is, if just one-third broke away. Parliament scrapped that in 2003, and today only one exception survives, a "merger" of two-thirds.This is the same arithmetic Raghav Chadha used. In April, Chadha and six other Aam Aadmi Party Rajya Sabha members — Sandeep Pathak, Ashok Mittal, Swati Maliwal, Harbhajan Singh, Vikramjit Singh Sahney and Rajinder Gupta — declared they were merging with the BJP. Seven of AAP's 10 Upper House members made two-thirds, and Chadha expressly argued that made them safe.The Rajya Sabha chairman accepted their logic; they kept their seats and are now seen in records as the BJP's. The AAP has, through another of its Rajya Sabha MPs, Sanjay Singh, has petitioned for their disqualification.The TMC rebels in the Lok Sabha are now reaching for the same legal lever.Caveats remain for Kakoli's groupBut the formula is harder to use than it looks, and Kakoli's group faces three problems.First, the exception protects you only if you merge with another party. Kakoli has said: "We have sought (from the sepaker) separate seating arrangements as a separate bloc." Former Lok Sabha Secretary General PDT Achary has told HT the two-thirds clause applies “only for merger”, adding there is “no way that the Speaker can recognise them as a separate group in the Lok Sabha”.Second, even the merger route is contested. After Chadha's switch, senior lawyer and MP Kapil Sibal argued it was not a true merger at all — his point being that two parties must merge first before their legislators can move, which did not happen. The question is expected to be settled by the Supreme Court in an ongoing matter about Goa, for instance. That may take time.Third, and most importantly as of June 9 afternoon, the numbers are in dispute. Kakoli insists “nearly 20 TMC MPs, including me, have decided to support the NDA”. Mamata Banerjee's loyalists say that figure is a stretch. Bardhaman-Durgapur MP Kirti Azad called it "the fake and fabricated narrative of the dirty tricks department of BJP," saying only 13 MPs — 12 from the Lok Sabha and one from the Rajya Sabha — attended the rebel meeting, and “no one else has signed on the dotted line apart from these”. Dum Dum MP Saugata Roy, who says he turned down an offer to switch, predicted the rebels would find it “very difficult” to reach two-thirds.The loyalists have argued against the switch even if the number were to be reached. Krishnanagar MP Mahua Moitra said the rebels won in 2024 on TMC ticket. "Mandate was NOT for NDA," she wrote on X, daring them to “resign your seats and contest on BJP ticket”. In her trademark use of colourful language, she called them “traitors with yellow-stained pants”.Kakoli Ghosh has been defiant, though so far she has not shown a letter that carries names and signatures she claims to have. "Mera sar katega lekin jhukega nahi [My head may be severed, but it will not bow]," she said in Hindi, insisting she has “fought for Bengal” for 40 years and is acting on the state's interests, not her own.How numbers moved in assemblyThere is an irony in all this, and that involves Chadha again. Back in 2022, as the youngest member of the Rajya Sabha and still a loyalist of Arvind Kejriwal, Chadha himself had moved a bill seeking to make defection harder. It called for raising the merger bar from two-thirds to three-fourths, and banning defectors from contesting for six years. That was a private member's bill, which rarely go beyond being tabled.There is a more recent precedent, though, staring at Mamata Banerjee as her party comes apart within weeks of losing power to the BJP after 15 years ruling West Bengal. She is facing rebellion at the state level too, and in the assembly the rebels already have the numbers the parliamentary group lacks.The TMC has 80 MLAs there, so two-thirds — the bar for the merger exception — works out to 54. The 58 legislators who broke away to back the expelled Ritabrata Banerjee were well past that line, thus enough to install him as Leader of the Opposition over the leadership's nominee.