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US Democrats have ruled out a “witness swap” with Republicans in President Donald Trump’s impeachment trial.
Lawmakers who are seeking to remove the president from office hope to hear testimony from his former National Security Adviser John Bolton.
But Democrats refused any deal to allow the son of former US Vice-President Joe Biden to be called as a witness.
The trial could end next week, but Mr Trump’s fellow Republicans control the Senate and are unlikely to oust him.
Attending the World Economic Forum in Davos, Switzerland, on Wednesday, Mr Trump jokingly warned he might confront Democrats by coming to “sit right in the front row and stare at their corrupt faces”.
Mr Trump is accused of abuse of power and obstruction of Congress.
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He denies using US military aid as a bargaining chip in an attempt to prod Ukraine into announcing an investigation to discredit his would-be Democratic White House challenger, Mr Biden.
Mr Trump has been touting corruption claims against Mr Biden, whose son Hunter held a lucrative board position with a Ukrainian gas firm while his father was US vice-president and in charge of American-Ukrainian relations.
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What is the witness swap proposal?
Democrats want to call Mr Bolton, who referred to the White House’s alleged political pressure on Ukraine as a “drug deal”, according to previous witness testimony in the House of Representatives.
But the former national security adviser has said he will not consider testifying unless served with a legal summons known as a subpoena.
Mr Trump’s Republican allies have argued Hunter Biden should also be ordered to appear before the impeachment trial.
But Senator Chuck Schumer of New York, the top Democrat in the Senate, told reporters during a break in the trial on Wednesday: “That trade is not on the table.”
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“This isn’t like some fantasy football trade,” he told reporters. “Trials aren’t trades for witnesses.”
Joe Biden said on Wednesday in Osage, Iowa, where he is campaigning for the White House that he would not offer himself up in any witness trade.
“We’re not going to turn it into a farce or political theatre,” Mr Biden said. “I want no part of that.”
Defending his son, Mr Biden added: “There’s no body that’s indicated there’s a single solitary thing he did that was inappropriate or wrong – other than the appearance. It looked bad that he was there.”
Mr Biden said last year that if elected president, no-one in his family would hold a job or have a business relationship with a foreign corporation.
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What’s happening in the trial?
House Democrats have up to three days to make their case as they present their arguments in the Trump impeachment trial in the Senate.
Mr Trump’s defence team will have three days after that for a rebuttal.
On Wednesday, the lead Democratic prosecutor, California congressman Adam Schiff, urged Republicans to vote to remove Mr Trump from office to “protect our democracy”.
Mr Schiff, who is chairman of the House Intelligence Committee, warned that senators would “also undermine our global standing” if they do not oust the president.
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The first day of the trial dragged on till the early hours of Wednesday morning as the senators debated a flurry of incremental motions.
Much of the evidence being laid out is a rehash of testimony already presented exhaustively in the House of Representatives, which voted to impeach Mr Trump last month.
Under arcane rules, senators are forbidden to drink coffee on the chamber floor and are only allowed water and milk.
Several members of the chamber have been spotted dozing during the proceedings.
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