NEW DELHI: India's opposition parties Wednesday mounted a united attack on Prime Minister Manmohan Singh after he denied any knowledge of an alleged cash-for-votes scandal. The opposition targeted Singh after US cables released by WikiLeaks last week said his government had allegedly paid cash to lawmakers to win a key 2008 confidence vote for a nuclear technology deal with the United States. “The prime minister has to take responsibility (for the scandal), he is the head of the government,” said Sushma Swaraj, leader of the main opposition Hindu nationalist Bharatiya Janata Party. “There is a cloud of suspicion, the prime minister should come clear,” said Communist Party of India (CPI) lawmaker Gurudas Dasgupta. The opposition's attack comes at a time when the Congress-led government is already struggling against a mountain of corruption scandals. US diplomatic messages released by WikiLeaks to The Hindu newspaper reported that Nachiketa Kapur, identified as an aide to prominent Congress figure Satish Sharma, allegedly said $2.5 million had bought the votes of four MPs. A US embassy staffer in New Delhi was shown “two chests containing cash” and was also told that $25 million was “lying around” to ensure the government would survive the vote, according to the leaked cables. The 2008 vote has long been the subject of corruption claims. At the time, opposition MPs in parliament waved wads of money they alleged the government was using to try to bribe them. “What happened in 2008 was a fraud on India's democracy,” said Arun Jaitley, leader of the BJP in the upper house. The BJP earlier called for the prime minister to quit over the scandal. Singh, whose “Mr Clean” image has come into question as he tries to shrug off the corruption scandals, stated in parliament last week that he was not aware of any acts of purchasing votes. Pushing through the deal, which ended energy-hungry India's civilian nuclear isolation, has been billed as Singh's greatest political triumph.