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. That is from former NSA 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.

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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.

Here is what to watch for as the Senate impeachment trial resumes on Monday at 1pm local time (18:00 GMT):

Defence resumes arguments

After a two-hour opening argument on Saturday, Trump's defence team will lay out its case in depth beginning Monday. White House counsel Pat Cipollone said Trump's lawyers do not expect to take the full 24 hours allotted to them, but there will be arguments from some familiar faces.

Harvard law professor emeritus Alan Dershowitz, former independent counsel Ken Starr and former Florida Attorney General Pam Bondi will speak on specific topics.

Dershowitz said on 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 on 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. An extended focus on Joe Biden, one of the leading Democratic presidential contenders, could mean blowback from even some of the GOP members of the Senate.

Question time

Once 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.

Chief Justice John Roberts will read the questions aloud. He is expected to alternate between both sides of the aisle. Many senators have been taking copious notes throughout the trial in preparation for the question-and-answer time.

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.

Witnesses

Under 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 Bolton, and Trump's 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. Still, Bolton has said he would appear if issued a subpoena by the Senate.

Trump speaks as then-NSA Bolton listens during a meeting with South Korean President Moon Jae-in [File: Oliver Contreras/EPA]

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 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 has been looking for as soon as the end of the week.

Senate rules also call for four hours of deliberations before voting. Since senators are required to sit silently during the trial, expect a closed session where they can deliberate in private.

A new tape

Trump's lawyers argued on Saturday that no one knows what his motives were on holding up military assistance to Ukraine. A recording obtained by the AP hours later suggests the president well understood that assistance was a point of leverage over Ukraine.

The recording is of 2018 meeting at the Trump Hotel in Washington, DC, that Trump had with donors, including two now-indicted associates of his personal lawyer, Rudy Giuliani. The audio portion includes Trump inquiring about Ukraine, "How long would they last in a fight with Russia?" He later calls for the firing of the US ambassador to Ukraine, Marie Yovanovitch.

The recording contradicts the president's statements that he did not know the Giuliani associations, Lev Parnas and Igor Fruman. They are key figures in the investigation who were indicted last year on campaign finance charges.

If new evidence and witnesses are allowed, the recording could take centre stage in the Senate proceedings.

Election

The trial is resuming with one week to go until the Iowa caucuses, and is again keeping four Democratic contenders - Senators Bernie Sanders, Elizabeth Warren, Michael Bennet and Amy Klobuchar - in Washington, DC, instead of campaigning at a critical point in the race.

While they are trapped in Washington, DC, Trump will venture outside the capital as he seeks both to exert political retribution on Democrats who impeached him and reward a party-switching legislator who backed him in the House.

President Trump reacts after speaking during a campaign rally in Toledo, Ohio [File: Tony Dejak/AP Photo]

Trump will hold a rally on Tuesday in New Jersey to repay the favour to Representative Jeff Van Drew, who became a Republican last month after voting against the articles of impeachment as a Democrat. He is also set to appear in Iowa on Thursday, days before the caucuses.

Meanwhile, Trump is already looking ahead to his likely acquittal, whenever it may come, promising that Democrats will face consequences for trying to remove him from office. "Shifty Adam Schiff is a CORRUPT POLITICIAN, and probably a very sick man," Trump tweeted on Sunday. "He has not paid the price, yet, for what he has done to our Country!"

Schiff is the chairman of the House Intelligence Committee and lead impeachment manager. Asked on NBC's Meet the Press programme whether he viewed the tweet as a threat, Schiff replied, "I think it's intended to be.''