A senior US diplomat directly implicated US President Donald Trump on Wednesday in a scheme to force Ukraine to probe a political rival, in bombshell testimony to a televised impeachment hearing.Gordon Sondland, the ambassador to the European Union, told lawmakers he followed the president's orders in seeking a "quid pro quo" deal for Ukraine to investigate Democrat Joe Biden in exchange for a White House summit.Sondland said Trump's personal lawyer Rudy Giuliani led the effort at Trump's direction to pressure Ukraine President Volodymr Zelensky for the investigation and that top officials in the White House and State Department knew about it.The unexpectedly damning testimony drew a sharp backlash from Trump who tweeted: "This Witch Hunt must end NOW. So bad for our Country!"Trump said he barely knew Sondland and had not spoken to him much, despite the senior diplomat having donated $1m to his inauguration and testifying that he had spoken to the president some 20 times while ambassador.'Extortion'Democrats said Sondland's seven hours of testimony before the House Intelligence Committee had bolstered their case for Trump's impeachment for what they have labelled "extortion"."Today's testimony is among the most significant evidence to date," said committee chairman Adam Schiff."It goes right to the heart of the issue of bribery as well as other potential high crimes or misdemeanours."A succession of Democrats hoping to win the nomination to take on Trump in next year's election also said the testimony had strengthened the case for impeachment as the issue dominated the opening exchanges in their latest televised primary debate.Sondland said Trump directed him and two other senior diplomats to work with Giuliani.From early in the year, Giuliani mounted a pressure campaign on Zelensky's government to investigate Biden over his son Hunter's ties to a Ukrainian energy company, Burisma, and to probe a conspiracy theory espoused by Trump that Ukraine helped Democrats against him in 2016.Biden is one of the favourites to challenge Trump in next year's presidential election."Mr Giuliani demanded that Ukraine make a public statement announcing the investigations of the 2016 election/DNC server and Burisma," Sondland told the panel."Mr Giuliani's requests were a quid pro quo for arranging a White House visit for President Zelensky."Top officials informedFar from being a "rogue" operation outside normal US diplomatic channels, Sondland told the hearing top officials - including Vice President Mike Pence and Secretary of State Mike Pompeo - were kept constantly informed."We followed the president's orders," he said.Sondland, who like Trump is a multimillionaire developer with a chain of high-end hotels, wore a $55 000 Breguet white gold watch to the hearing as he fended off pressure from both Democrats and Republicans.He had not implicated the president in earlier private testimony, when he answered scores of questions by saying he could "not remember".But subsequent testimony by other witnesses which had further implicated him in the Ukraine pressure scheme had jolted his memory, he said on Wednesday.While he confirmed the linkage between the investigations and a White House meeting between Zelensky and Trump, he would not attest to allegations that Trump froze $391m in aid as well to Ukraine to add pressure on Ukraine.