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Mallikarjun Kharge, widely seen to be the "Gandhi-approved" candidate, won 90 per cent of the votes cast on Monday. He scored 7,897 votes while Shashi Tharoor finished with 1,072.
Even before the results were announced, Rahul Gandhi had declared Mr Kharge as the winner at a press conference in Karnataka.
Shashi Tharoor alleged "extremely seious irregularities" in the election process. Salman Soz, representing Mr Tharoor's team, later said they had been “assured of a fair inquiry” had had agreed that counting should continue.
The counting, which began at 10 am at the Congress headquarters in Delhi, ended around 1 pm.
Around 96 per cent of the 9,915 eligible leaders had voted in the election held on Monday, the Congress had said.
Mr Kharge and Mr Tharoor both maintained that the Gandhis are neutral about the election. Mr Tharoor, however, has said that the "odds have been stacked against us as the party leaders and establishment were overwhelmingly with the other candidate".
The election comes three years after Sonia Gandhi agreed to temporarily lead the party when Rahul Gandhi stepped down, taking responsibility for the two consecutive routs of the party in 2014 and 2019 general elections.
It was held after multiple calls for change and hiccups over finding a candidate against Shashi Tharoor. Mr Kharge was a last-minute entrant, persuaded to contest by a section of Central leaders when Rajasthan Chief Minister Ashok Gehlot dropped out of the race even before it started.
Seen initially as the front-runner, Mr Gehlot's candidature went sideways after his loyalist MLAs launched an open revolt in Rajasthan to keep his arch-rival Sachin Pilot from succeeding him as the Chief Minister.
Since Independence, the Congress has mostly been led by a member of the Gandhi family, who were elected unanimously. Elections were held only six times as there was more than one candidate - starting in 1939 when P Sitaramayya, backed by Mahatma Gandhi, lost to Netaji Subhas Chandra Bose.
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