Upping the ante two days after his sharp outburst, Singh put out a video message to attack Modi over his remarks insinuating that the Congress leader colluded with Pakistan in the Gujarat polls, and said it was unfair to question the nationalism of public servants.
"I am deeply pained and anguished by the falsehood and canards being spread to score political points in a lost cause by none less than Prime Minister Narender (sic) Modi. Fearing imminent defeat in Gujarat, desperation of Prime Minister to hurl every abuse and latch on to every straw is palpable," he said.
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"I sincerely hope that he will apologize to the nation for his ill thought transgression to restore the dignity of the office he occupies," he said.
Strongly rejecting the "innuendos and falsehoods" made by Modi, Singh said he did not discuss Gujarat elections with anyone else at the dinner hosted by Congress leader Mani Shankar Aiyar as alleged by Modi and nor was the Gujarat issue raised by anyone else present at the dinner.
"The discussion was confined to Indo-Pakistan relations. It is wrong and unfair to question nationalism of journalists, diplomatics and public servants present at the dinner," he said.
The former premier also asserted that Modi "is setting a dangerous precedent by his insatiable desire to tarnish every constitutional office, including that of a former prime minister and Army Chief", and described it as sad and regrettable.
The Congress Party needs no sermons on "nationalism" from a party and prime minister, whose compromised track record on fighting terrorism is well known, he added.
"Let me remind Narender (sic) Modi that he had gone to Pakistan uninvited after the terrorist attacks in Udhampur and Gurdaspur. Let him also tell the country the reason for inviting the infamous ISI of Pakistan to our strategic Air Base in Pathankot to investigate a terror attack that emanated from Pakistan," he said.
He also said that his track record of public service to the country over last five decades is known to everyone and no one including Modi, can "lamely" question it to gain lost political ground.