US Democrats have been dealt a major blow in their efforts to call witnesses at President Trump's impeachment trial. They needed four Republicans to vote with them to allow witness testimony, but one of the few wavering senators said he would not support the measure. Lamar Alexander said the Democrats had proved Mr Trump acted inappropriately but it was not an impeachable offence. The announcement paves the way for the possible acquittal of the president by the Senate as early as Friday. The Democrats had especially wanted to call former National Security Adviser John Bolton who reportedly said Mr Trump told him directly that he was withholding US military aid to Ukraine until it agreed to investigate his rival, Joe Biden. What did Lamar Alexander say? In a statement late on Thursday after a long question-and-answer session at the Senate, Mr Alexander, who represents Tennessee, said the Democrats had proven that Mr Trump's actions were "inappropriate". But the 79-year-old said: "There is no need for more evidence to prove something that has already been proven and that does not meet the United States Constitution's high bar for an impeachable offence." He added: "The question then is not whether the president did it, but whether the United States Senate or the American people should decide what to do about what he did. I believe that the Constitution provides that the people should make that decision in the presidential election that begins in Iowa on Monday." Democrats were hoping that four Republican senators - Mitt Romney, Lisa Murkowski and Susan Collins, as well as Mr Alexander - would enable them to reach the 51 votes necessary to call witnesses. On Thursday, Ms Collins joined Mr Romney by saying she would also vote for more witnesses. Mr Alexander's announcement is a sign that Republicans will be able to block the move and put an end to Mr Trump's trial with his expected acquittal. A two-thirds majority in the Senate is required to remove him from office, and Republicans hold a 53-47 majority. Each side is expected to present closing arguments in Friday's session, before the Senate votes on hearing witnesses. If the vote were to end in a tie, it would mean that the motion had failed unless US Supreme Court Chief Justice John Roberts, who is presiding over the trial, decided to break it, which is unlikely.