The only seat that promised a tough contest in the Bengal bypoll was Dinhata in Cooch Behar district, considered the bastion of Union minister of state for Home Nisith Pramanik. The Trinamool fielded Udayan Guha, an old political warhorse, and he trounced BJP's Ashok Mondal by 1.63 lakh votes. So what changed the ruling party's fortunes in a matter of months?