US President Donald Trump's legal team concluded oral arguments on Tuesday in his impeachment trial in the Senate, arguing that Democrats are seeking to remove him from office over policy differences and dismissing the need for further testimony from witnesses.

Trump's lawyers Pat Cipollone, Jay Sekulow and Pat Philbin spoke in a session that ended by midafternoon, well before the allotted time for their opening arguments had expired.

Cipollone wrapped up his presentation by saying: "End the era of impeachment for good. You know it should end."

He warned that lowering the bar for impeachment and removal from office would set a historic precedent that would leave future presidents vulnerable to a Congress led by a different party.

He argued that the accusations in the two articles of impeachment from the Democrat-controlled House of Representatives-abuse of power and obstruction of Congress-were too vague to justify removing a president from office. "They fall far short of any constitutional standard and they are dangerous," he said.

To make his point, he played a series of videos from Democrats decrying the impeachment of thenpresident Bill Clinton in 1998.

Sekulow 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."

The trial was to resume Wednesday afternoon with a question period for senators. Questions will alternate between the majority and minority for up to eight hours on Wednesday and again for up to eight hours on Thursday.

They are submitted to Chief Justice John Roberts, who reads them out. Roberts asked the managers and Trump counsel on Tuesday to abide by a time limit of five minutes or less to answer each question.

After that there will be as much as four hours of debate on whether to subpoena witnesses or documents.

The Senate would then proceed to vote on whether to hear from John Bolton, Trump's former national security adviser, subpoena notes he took during his tenure, and gather other information or hear from other witnesses.

The New York Times reported on Saturday that Bolton claims in an unpublished manuscript that Trump said he wanted to freeze military aid for Ukraine until officials there announced an investigation of former vice-president Joe Biden and his son Hunter. The book is expected to be published in March.

The center of the case for Trump's impeachment is the allegation that he withheld military aid and a White House meeting to pressure Ukraine to investigate the Bidens.

Trump's attorneys have argued that he was justified in seeking investigations because of a history of corruption involving a company connected with Hunter Biden. The younger Biden served on the board of Burisma, a Ukrainian energy company, while his father was vice-president overseeing US efforts to combat corruption in Ukraine.

No evidence has emerged of wrongdoing by either Biden.

Sekulow addressed the Bolton news report, saying: "You cannot impeach a president on an unsourced allegation." He argued that the revelations from Bolton's leaked manuscript wouldn't be admissible during a typical trial.

He read several statements denying Bolton's allegation from Trump, the Department of Justice and the chief of staff to Vice-President Mike Pence.