After months of amassing debt of Energy Future Holdings Corp., Elliott bought a strategic slice of notes that would ensure its ability to block the deal, people familiar with the matter said.
A deal could be announced as soon as Monday.
Elliott Management Corp., a hedge fund, is Energy Future's biggest creditor and has indicated it will sign off on Sempra's offer, according to sources cited by Reuters.
Buffett is famous for refusing to take part in bidding wars and the Wall Street Journal reported Sunday that Sempra had sweetened its initial bid of $9.3 billion for Oncor.
San Diego, California-based Sempra has made a $9.3 billion offer for the Texas utility, said the people, who declined to be identified because the matter is not public.
Mr Buffett and Greg Abel, Berkshire Hathaway Energy chairman and chief executive, said in a statement on Thursday that they stood by their agreement to buy Oncor and did not plan to raise their offer.
Two other offers prior to Berkshire Hathaway had been blocked by regulators in Texas.
EFH, which was formerly known as TXU, was infamously acquired by KKR, TPG Capital and the buyout arm of Goldman Sachs in 2007 for $45 billion in what turned out to be the largest leveraged buyout in history.
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