Ghulam Nabi Azad had been a leading member of the G-23 or the group of 23 "dissenters".
Ghulam Nabi Azad, one of the Congress's tallest leaders, quit the party on Friday with a scalding attack on Rahul Gandhi, blasting him for "childish behaviour", "glaring immaturity" and for letting a "coterie of inexperienced sycophants" run the party.
In a lacerating five-page resignation letter to Sonia Gandhi, he blamed her son Rahul for the Congress's defeat in the 2014 national election - a turning point for the party that has been struggling to win elections since.
The Congress is "comprehensively destroyed" and has reached a point of no return, he declared, and slammed a "remote control model" in which Sonia Gandhi is "just a nominal figurehead" while important decisions are taken by "Rahul Gandhi or rather worse his security guards and PAs (personal assistants)".
Mr Azad, 73, is the senior most leader to quit the Congress, which has seen several big exits over the past few years. There are reports of another senior leader exiting soon.
Honoured with the Padma Bhushan this year, Mr Azad has been accused by a section of the Congress of cozying up to the BJP and Prime Minister Narendra Modi, who describes him as a good friend.
Mr Azad, a Congress man for over five decades, dropped the bombshell at a time all three Gandhis are abroad.
In his parting shot, Mr Azad said the Congress had conceded its political space to the BJP and regional parties "because the leadership in the past eight years has tried to foist a non-serious individual" at the helm of the party.
"Unfortunately, after the entry of Rahul Gandhi into politics and particularly after January 2013, when he was appointed Vice President by you, the entire consultative mechanism which existed earlier was demolished by him," Mr Azad wrote, pointing out that Sonia Gandhi had heeded senior leaders.
All senior and experienced leaders were sidelined, he said, and "new coterie of inexperienced sycophants" started running the affairs of the party.
"One of the most glaring examples of his immaturity was the tearing up of the government ordnance in the full glare of the media by Rahul Gandhi," he said.
Rahul Gandhi in 2013 tore up a controversial ordnance or special order to spare convicted lawmakers from disqualification and called it "complete nonsense", embarrassing his own Congress-led government and then Prime Minister Manmohan Singh.
"This childish behaviour completely subverted the authority of the Prime Minister and the government of India. This one single action more than anything else contributed significantly to the defeat of the UPA government in 2014 that was at the receiving end of a campaign of calumny and insinuation from a combination of forces of the right wing and certain unscrupulous corporate interests," Mr Azad's letter said.
Mr Azad went on to chronicle the Congress's downspiral over the past few years, pulling no punches.
"Under your stewardship since 2014 and subsequently that of Rahul Gandhi, the Congress has lost two Lok Sabha elections in a humiliating manner. It has lost 39 out of the 49 assembly elections held between 2014 - 2022. The party only won four state elections and was able to get into a coalition situation in six instances. Unfortunately, today, the Congress is ruling in only two states and is a very marginal coalition partner in two other states."
Mr Azad said since the 2019 national election the situation has only worsened. "After Rahul Gandhi stepped down in a 'huff' and not before insulting all the senior party functionaries who have given their lives to the party in a meeting of the extended working committee, you took over as interim president. A position that you have continue to hold even today for the past three years."
The former Jammu and Kashmir Chief Minister said he joined the Congress in the 1970s, when it was still taboo to be associated with the party in his home state.
Mr Azad was a leading member of the G-23 or the group of 23 "dissenters" who wrote to Sonia Gandhi in 2020 calling for a complete overhaul of the organization and a full-time, collective and visible leadership.
He said after that letter, he was attacked, vilified and humiliated.
The Congress, Mr Azad said, has lost both the will and the ability "under the tutelage of the coterie" that runs it to fight for what is right for India.
Calling the upcoming polls for a new chief a "giant fraud", he said the "chosen one" would be nothing more than a puppet on a string.
The former Rajya Sabha member said instead of a "bharat Jodo" campaign - Rahul Gandhi will travel the country - the leadership should have gone for a "Congress Jodo" exercise.
The Congress said it was "most regrettable" that a senior leader quit just before the party's big "Bharat Jodo" campaign, which would be addressed by Rahul Gandhi.