Prime Minister flagged off his West Bengal Assembly poll campaign officially today by lashing out against Chief Minister Mamata Banerjee and promising a "Sonar Bangla" (Golden Bengal) if the BJP is voted to power. He attacked the state government for allegedly letting down the people who had placed much expectation in it. "Bengal trusted Didi for Pariborton. She broke the trust," he said today at a massive Kolkata rally.