The letter refers to a speech PM Modi gave while campaigning for the Karnataka assembly election. On May 6, while speaking in Karnataka's Hubli, PM Modi had said, "Congress ke neta kaan khol ke sun lijeye, agar seemayo ko paar kaorogi, toh yeh Modi hai, lene ke dene pad jayenge (Congress leaders should listen to me with open ears, if you cross your limits, this is Modi, you will have to pay)."
The Congress leaders' letter mentions this quote and says that the "threat held out by the prime minister to the [Congress's] leadership deserves to be condemned. This cannot be the language of the prime minister of a constitutionally governed democratic country of 1.3 billion people."
"Such discourse whether in public or private is unacceptable conduct. The words used are menacing and intimidating with [an] intent to insult and provoke breach of the peace," the letter further says.
Apart from Manmohan Singh, the letter has been signed by Mallikarjun Kharge, P Chidambaram, Ambika Soni, Anand Sharma, Motilal Vora, Karan Singh, Ahmed Patel and Kamal Nath among others.
The letter begins with the oath that the prime minister takes on assuming office and then goes on to say, "In the past, all prime ministers of India have maintained immense dignity and decorum in discharge of public or private functions/actions. It is unthinkable [that the PM] would utter words which are threatening, intimidating in content and a public warning to the leaders and members" of the Congress.
"[The] President may caution the prime minister from using such unwarranted, threatening and intimidating language against leaders of the Congress Party of any other party or person as it does not behove the position of the prime minister," the leaders sign off.
This is not the first time PM Modi's comments from an election campaign have sparked a political controversy. During the Gujarat election, the Congress had similarly taken offence at the prime minister suggested that Manmohan Singh and other party leaders were in touch with Pakistani officials ahead of the Gujarat polls.
That had led to a Parliament stalemate with the Congress demanding that PM Modi apologise on the floor of the house. The issue was finally resolved after Finance Minister Arun Jaitley tendered a half-apology.