US President Donald Trump's impeachment defence team will begin their second day of arguments on Monday, following three days of presentations by the Democrats making the case against the president. On Saturday, Trump's defence team offered only a "preview" to its line of defence, apparently responding to the ratings-minded president's complaints that the first day of the US weekend is the "Death Valley" of television viewing. In just two hours of arguments, Trump's lawyers argued that Democratic House of Representative managers prosecuting the case had not presented a full picture to senators while framing the impeachment as an attempt to undermine American democracy. More: Trump impeachment: US Senate approves trial rulesWhat evidence has come out since Trump was impeached?What happened in the Senate impeachment trial of Bill Clinton?The seven Democrat House managers had previously been given 24 hours of arguments over three days to make their case. Trump's team will have the same amount of time. As Trump's defence team prepares for what is expected to be a much longer second day of proceedings, here are all the latest updates as of Monday, January 27:Romney: 'Increasingly likely' at least four senators will vote for Bolton's testimonyRepublican Senator Mitt Romney said on Monday that it was "increasingly likely" at least four Republican senators would join Democrats in calling for John Bolton, the former White House national security adviser, to testify in the Senate impeachment trial of Trump.A simple majority is needed for such a measure to pass. Bolton had reportedly written in an unpublished manuscript that Trump had told him in August of last year that he wanted to keep withholding military aid from Ukraine until the country agreed to help investigate his Democratic rivals. Bolton has said he will testify if subpoenaed. Senator Mitt Romney has said it's "increasingly likely" four Republicans will vote in favour of subpoenaing Bolton.No WH personnel outside of NSC have reviewed Bolton manuscript: SpokesmanThe spokesman for the National Security Council has said that no other White House personnel have reviewed a manuscript by former National Security Advisor John Bolton. In the draft of his upcoming book, Bolton reportedly recounts that Trump told him he wanted to withhold military aid to Ukraine until the country agreed to launch investigations into the president's political rivals. Bolton had submitted the draft to the White House for a standard review process, the New York Times reported. The review may have given insights into Bolton's possible testimony to Trump's defence team, the newspaper noted. Schiff: Trump tweets intended to be a threatThe Senate impeachment trial took a break on Sunday, but tweets from the president did not. Trump again claimed there was no evidence of wrongdoing presented by Democrats, derided those prosecuting the case against him, and claimed he never told former National Security Adviser John Bolton that he was withholding military aid to Ukraine in exchange for politically motivated investigations. In one tweet, the president tweeted that "Shifty Adam Schiff is a CORRUPT POLITICIAN, and probably a very sick man. He has not paid the price, yet, for what he has done to our Country!"Asked later on NBC whether he considered the tweet as a threat, the California lawmaker said he thought it was "intended to be".Who is on Trump's defence team?Trump's defence team will begin their second of three days of arguments on Monday. Read more about the key players in the trial here. Who are the House managers? The House of representatives designated seven Democrats to present the case against Trump, which they concluded last week. Bolton to testify? On Sunday, The New York Times reported that Trump said he wanted to maintain a freeze on military assistance to Ukraine until it aided political investigations into his Democratic rivals - that according to former national security adviser John Bolton in a draft of his forthcoming book.The report, which was later confirmed by The The Associated Press news agency, challenges the defence offered up by Trump and his lawyers that the freeze in aid was related to a general fight against corruption in Ukraine. Under the rules resolution passed at the beginning of the trial, debate on whether additional witnesses can be subpoenaed will begin after the defence finishes their three days of arguments and the Senators are given 16 hours for questions. In light of revelations, that debate will be particularly lively, especially since Bolton has said he will testify if subpoenaed. Read more on what to expect this week here. Former National security adviser John Bolton has said he is prepared to testify if subpoenaed.Trump tapes, new evidence? A new recording has emerged of a 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 didn't 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.As Senators debate whether new evidence and witnesses will be allowed in the trial, the recording could take centre stage in the proceedings. What has happened so far in the trial? The impeachment trial of Trump began in earnest last week, after a ceremonial start on January 16 that saw the swearing in of Chief Justice John Roberts, who is presiding over the proceedings, and the 100 members of the Senate. On early Wednesday morning, the Senate voted along partisan lines to approve Senate Majority Leader Mitch McConnell's rules resolution after nearly 12 hours of debate. Meanwhile, 11 amendments introduced by Democrats were blocked. Read more about that day here. The Democratic House managers then presented their arguments for three days, followed by Trump's defence's first arguments on Saturday. Catch up on the live blogs from Wednesday, Thursday , Friday and Saturday.