Image copyright Getty Images Image caption President Trump's personal lawyer Jay Sekulow said impeachment was a danger to the nation

Donald Trump's defence team concluded its oral arguments in the US Senate impeachment trial on Tuesday, setting the stage for two days of questioning.

The closing arguments came amid a bombshell report that former national security adviser John Bolton implicates President Trump in his new book.

Mr Bolton reportedly writes that Mr Trump directly withheld security aid to Ukraine for his own political benefit.

The report added weight to the Democratic Party's call for witnesses.

The president's Republican Party has tried to resist calls for witnesses to testify, largely out of concern over what Mr Bolton might say. Four Republicans would need to side with Democrats in a vote on whether new testimony will be heard.

US media outlets reported on Tuesday that Mitch McConnell, the most senior Republican, had told his senators during a closed-door meeting that following the Bolton reports the party did not have the votes to hold off witnesses.

But Mr McConnell and his leadership team were reportedly confident of pressuring enough Republican senators by the end of the week to win a vote.

Defence rests

The president's defence wrapped up its arguments early on Tuesday, having used around half of its allotted 24 hours over three days. Their approach was a contrast with that of the Democrats, who used all of their allotted time to present a detailed case against the president.

Jay Sekulow, the president's personal lawyer, said: "The bar for impeachment cannot be set this low. Danger, danger, danger. These articles must be rejected. The Constitution requires it. Justice demands it."

White House counsel Pat Cipollone concluded by calling on senators to "end of the era of impeachment for good".

Media playback is unsupported on your device Media caption Mitt Romney: 'Important to hear from John Bolton'

President Trump was impeached on charges of abuse of power and obstruction of Congress. He is accused of withholding $391m (£300m) in military aid, in an attempt to pressure Ukraine's President Volodymyr Zelensky to announce a corruption investigation into Mr Trump's Democratic political rival, Joe Biden.

Mr Trump denies the allegations against him, and Republicans have argued that no first-hand witnesses have so far connected the president to a scheme to withhold aid for political benefit.

The significance of the apparent revelations from Mr Bolton, first reported by the New York Times, is that they would undermine that argument. He reportedly says in his forthcoming book that he was instructed directly by the president to withhold the aid in order to pressure Ukraine.

Mr Trump told reporters last week that he did not want Mr Bolton to testify, citing national security issues.

What happens next?

The Senate trial now moves into a two-day period of questioning, during which each party will alternate questioning for up to 16 hours throughout Wednesday and Thursday. The senators will not ask questions themselves but submit them for Chief Justice John Roberts, who is presiding, to read.

A vote on whether or not to call witnesses in the trial is expected on Friday.

Four Republicans would be required to vote with the Democrats to reach the necessary majority to call witnesses. Several more moderate senators appeared more prepared to do so in the wake of the Bolton book reports.

"I think it's increasingly likely that other Republicans will join those of us who think we should hear from John Bolton," said Senator Mitt Romney of Utah on Monday.

Maine's Susan Collins, a vulnerable Republican who is up for re-election this year, said the reports had prompted "a number of conversations among my colleagues".

Alaska Senator Lisa Murkowski said that Mr Bolton "probably has some things that would be helpful", but she did not confirm whether she would vote for witnesses.

On Monday, Republican Senator James Lankford proposed that lawmakers receive a copy of the manuscript of Mr Bolton's book "to see what they're actually saying". He was endorsed by staunch Trump ally, Senator Lindsey Graham of South Carolina.

Image copyright Getty Images Image caption Senator Lisa Murkowski of Alaska in the Capitol building on Tuesday

According to the latest non-partisan Quinnipiac University poll, 75% of voters believe witnesses should be allowed in the trial.

A two-thirds majority in the Senate is required to remove Mr Trump from office. With Republicans holding a 53-47 majority in the Senate, removal remains highly unlikely.

He is the third president in US history to be impeached and go on trial in the Senate.