US Senate Majority Leader Mitch McConnell said on Monday that he was not ruling out calling witnesses in President Donald Trump's impeachment trial - but indicated he was in no hurry to seek new testimony either - as politicians remain at an impasse over the form of the trial by the Republican-controlled Senate.The House voted on Wednesday to impeach Trump, who became only the third president in US history to be formally charged with "high crimes and misdemeanors". But the Senate trial may be held up until congressional leaders agree on how to proceed. Minority Leader Chuck Schumer is demanding trial witnesses who refused to appear during House committee hearings, including acting White House chief of staff Mick Mulvaney and former National Security Advisor John Bolton.More:Trump impeachment: As it happenedTrump impeachment: What happens next?Trump impeachment: Republicans, Democrats spar over next stepsMcConnell, who has all-but-promised a swift acquittal of the president, has resisted making any guarantees, and has cautioned Trump against seeking the testimony of witnesses he desires for fear of elongating the trial. Instead, he appears to have secured Republican support for his plans to impose a framework drawn from the 1999 impeachment trial of President Bill Clinton."We haven't ruled out witnesses," McConnell said Monday in an interview with "Fox and Friends." "We've said let's handle this case just like we did with President Clinton. Fair is fair."That trial featured a 100-0 vote on arrangements that established two weeks of presentations and argument before a partisan tally in which then-minority Republicans called a limited number of witnesses. But Democrats now would need Republican votes to secure witness testimony - and Republicans believe they have the votes to eventually block those requests.Democrats accuse Trump of abusing his power in office by pressuring Ukraine to investigate former Vice President Joe Biden, a leading candidate for the Democratic nomination to face Trump in the 2020 election. The impeachment inquiry was centred on a July phone call in which Trump asks his Ukrainian counterpart to investigate Biden. At the time of the call, the Trump administration was withholding nearly $400m in military aid from Ukraine. Trump was also impeached for obstruction of justice related to his refusal to participate in the inquiry. Trump has denied wrongdoing and branded his impeachment as a politically-motivated effort by Democrats to overturn the results of the 2016 election.Democrats reject Clinton modelIn a letter Monday to all Senators, Schumer argued that the circumstances in the Trump trial are different from that of Clinton, who was impeached after a lengthy independent counsel investigation in which witnesses had already testified numerous times under oath. Schumer rejected the Clinton model, saying waiting until after the presentations to decide on witnesses would "foreclose the possibility of obtaining such evidence because it will be too late".Schumer also demanded that the Senate, in addition to receiving testimony, also compel the Trump administration to turn over documents and emails relevant to the case, including the decision to withhold military assistance from Ukraine.In the letter, Schumer wrote that denying the Senate these documents "would be to turn a willfully blind eye to the facts" in the abuse of power and obstruction of Congress charges against Trump.Meanwhile, the White House is projecting confidence that it will prevail in a constitutional spat with Democrats. House Speaker Nancy Pelosi has delayed sending the articles of impeachment to the Senate in hopes of giving Schumer more leverage in talks with McConnell. But the White House believes Pelosi won't be able to hold out much longer."She will yield. There's no way she can hold this position," Marc Short, the chief of staff to Vice President Mike Pence, said Sunday. "We think her case is going nowhere.''The impasse between the Senate leaders leaves open the possibility of a protracted delay until the articles are delivered.Schumer told reporters in New York on Sunday that "the Senate is yearning to give President Trump due process, which means that documents and witnesses should come forward. What is a trial with no witnesses and no documents. It's a sham trial."Leverage?Trump has called the hold-up "unfair" and claimed that Democrats were violating the Constitution, as the delay threatened to prolong the pain of impeachment and cast uncertainty on the timing of the vote Trump is set to claim as vindication."Pelosi gives us the most unfair trial in the history of the US Congress, and now she is crying for fairness in the Senate, and breaking all rules while doing so," Trump tweeted Monday from his private club in Palm Beach, Florida, where he is on a more than two-week holiday vacation. "She lost Congress once, she will do it again!"Short called Pelosi's delay unacceptable, saying she's "trampling" Trump's rights to "rush this through, and now we're going to hold it up to demand a longer process in the Senate with more witnesses.""If her case is so air-tight ... why does she need more witnesses to make her case?'' Short said.White House officials have also taken to highlighting Democrats' arguments that removing Trump was an "urgent" matter before the House impeachment vote, as they seek to put pressure on Pelosi to send the articles of impeachment to the Senate. A close Trump ally, Senator Lindsey Graham said Pelosi would fail in her quest "to get Mitch McConnell to bend to her will to shape the trial". Graham is chairman of the Senate Judiciary Committee and was a House manager, comparable to a prosecutor, during the Senate's impeachment trial of Clinton."She'll eventually send the articles because public opinion will crush the Democrats,'' said Graham. Asked whether he expected witnesses in the Senate, he replied: : "No, I don't."At one point, Trump had demanded the testimony of witnesses of his own, like Democrats Joe Biden and his son Hunter, and the intelligence community whistle-blower whose complaint sparked the impeachment probe. But he has since relented after concerted lobbying by McConnell and other Senate Republicans who pushed him to accept the swift acquittal from the Senate and not to risk injecting uncertainty into the process by calling witnesses.The Senate's second-ranking Democrat, Dick Durbin of Illinois, said his party is looking for a signal from McConnell that he hasn't ruled out new witnesses and documents. But Durbin acknowledged that Democrats may not have much leverage in pushing a deal.He criticised both Republican and Democratic senators who have already announced how they will vote in the trial, saying the Constitution requires senators to act as impartial jurors. Republicans hold a 53-vote majority in the Senate."The leverage is our hope that four Republican senators will stand up, as 20 years ago, we saw in the impeachment of Bill Clinton, and say, this is much bigger than our current political squabbles," Durbin said.The Constitution requires a two-thirds majority in the Senate to convict in an impeachment trial - and Republicans have expressed confidence that they have more than enough votes to keep Trump in office.Short spoke on "Fox News Sunday", Durbin appeared on CNN's "State of the Union", and Graham was on Fox News Channel's "Sunday Morning Futures."