US President Donald Trump's impeachment trial enters a pivotal week as his defence team resumes its case and senators face a critical vote on whether to hear witnesses or proceed directly to a vote that is widely expected to end in his acquittal. The articles of impeachment charge Trump with abuse of power and obstruction of Congress.Those decisions on witnesses may be complicated by reports that Trump said he wanted to maintain a freeze on military assistance to Ukraine until it aided political investigations into his Democratic rivals, according to former national security adviser John Bolton in a draft of his forthcoming book. The report by The New York Times was later confirmed by The Associated Press news agency. The revelation challenges the defence offered up by Trump and his lawyers in his impeachment trial.The Capitol Hill manoeuvring will be complemented by high-stakes efforts on both sides of the aisle to claim political advantage from the proceedings as the presidential nominating season kicks off in Iowa on February 3.Defence resumes argumentsAfter a two-hour opening argument Saturday, Trump's defence team will lay out its case in depth beginning on Monday. White House counsel Pat Cipollone said Trump's lawyers don't expect to take the full 24 hours allotted to them, but there will be arguments from some familiar faces.READ | Trump is teases he could make an awkward, unscheduled appearance at his own impeachment trialHarvard law professor emeritus Alan Dershowitz, former independent counsel Ken Starr and former Florida Attorney General Pam Bondi will speak on specific topics.Dershowitz said Sunday he would argue that the charges against Trump are too minor to warrant the Republican president's removal from office under the Constitution. "Even if true, they did not allege impeachable offences," Dershowitz told "Fox News Sunday." The Trump team has also teased the notion that it would draw attention to Joe Biden and his son Hunter, who served on the board of a Ukraine gas company Burisma, while the elder Biden was vice president.Question timeOnce Trump's team concludes, senators will have 16 hours to ask questions of the House impeachment prosecutors and the president's legal team. Their questions must be in writing.ALSO READ | Democratic prosecutors to wrap up case against TrumpChief Justice John Roberts will read the questions aloud. He is expected to alternate between both sides of the aisle.Senator John Barrasso told reporters on Saturday that Republicans expected to get together on Monday to start formulating a list of questions."We will meet as a conference and decide what questions we want to pose, what the order may be of those of those questions," he said. WitnessesUnder the Senate rules passed last week, senators will get another chance to vote whether to consider new witnesses and evidence after the question and answer time is elapsed. Four Republicans would have to break ranks to join Democrats in the GOP-controlled Senate to extend the trial for an undetermined amount of time.If that happens, expect a bitter fight over which witnesses might be called and which documents might be subpoenaed. Democrats have called for testimony from Trump's former national security adviser, John Bolton, and his acting chief of staff, Mick Mulvaney. An attempt to call either probably would lead to a showdown with the White House, which claims both men have "absolute immunity" from being called to testify before the Senate, even in an impeachment trial.MUST READ | Democrats urge Republican 'courage' at Trump impeachment trialStill, Bolton has said he would appear if issued a subpoena by the Senate.While Republicans have hoped for a speedy trial, Trump has called for the testimony of the Bidens and the intelligence community whistle-blower whose complaint about Trump's July telephone call with Ukraine's leader instigated the impeachment inquiry.But some Republicans, including Republican Senator Lindsey Graham have expressed resistance to calling those witnesses. If the vote fails, the Senate could move swiftly to its vote on whether to remove or acquit Trump, giving the president the result he's been looking for as soon as the end of the week.Since senators are required to sit silently during the trial, expect a closed session where they can deliberate in private.