US President Donald Trump seems bound for acquittal in his impeachment trial after a closely watched Republican senator opposed calling more witnesses.

Photo: AFP

Democrats need four Republicans to vote with them to allow Senate testimony, but Lamar Alexander of Tennessee has said he will not support such a move.

Alexander said Democrats had proven Trump acted inappropriately, but it was not an impeachable offence.

Trump could now be cleared by the Senate as soon as Friday (local time).

The Democrats want to call former US National Security Adviser John Bolton, who reportedly said Mr Trump had told him directly he was withholding US military aid to Ukraine until it agreed to investigate his rival, Joe Biden.

What did Lamar Alexander say?

In a statement late on Thursday after a long question-and-answer session at the Senate, Alexander said the Democrats had demonstrated Mr Trump's actions were "inappropriate".

But the 79-year-old said: "There is no need for more evidence to prove something that has already been proven and that does not meet the United States constitution's high bar for an impeachable offence."

He added: "The question then is not whether the president did it, but whether the United States Senate or the American people should decide what to do about what he did.

"I believe that the constitution provides that the people should make that decision in the presidential election that begins in Iowa on Monday."

Democrats were hoping four Republican senators - Mitt Romney, Lisa Murkowski and Susan Collins, as well as Alexander - would defect and bring them over the 51-vote threshold needed to summon witnesses. On Thursday, Collins joined Romney by saying she would also vote for testimony.

Alexander's announcement is a sign that Republicans will be able to block the move and put an end to Trump's trial with his expected acquittal. A two-thirds majority in the Senate is required to remove him from office, but the president's fellow Republicans hold a 53-47 majority.

Each side is expected to present closing arguments in Friday's session, before the Senate votes on hearing witnesses. If the vote were to end in a tie, it would mean the motion had failed unless US Supreme Court Chief Justice John Roberts, who is presiding over the trial, decided to break it, which is deemed unlikely.

Why is Bolton so important?

A report in the New York Times earlier this week said Bolton had written in his forthcoming book that the president told him directly military aid was being withheld from Ukraine in exchange for the opening of an investigation into a Democratic political rival - the key impeachment charge against the president.

The report reinvigorated Democrats' attempts to call new trial witnesses.

If the reports about Bolton were true, and he were to testify to that effect, he would be the first witness in the process to directly link the president to an alleged quid pro quo (exchange of favours) with Ukraine and an abuse of presidential power.

Trump's lawyer expanded the defence in the Senate earlier this week. He suggested that anything a president did in service of his own re-election could be considered to be in the public interest, and therefore not impeachable. The argument shocked Republicans and Democrats alike.

The White House pushed back against the publication of Bolton's book, citing security concerns. The National Security Council alleged that the book had "top secret" details that must be removed, a claim Bolton rejects.

Bolton's lawyer, Charles Cooper, responded to the NSC letter last week by saying the book contained nothing classified as top secret.

"We do not believe that any of that information could reasonably be considered classified," Cooper wrote in an email to the White House on 24 January, the Washington Post reported.

Cooper also said he had asked for an expedited review of a chapter about Ukraine, adding that Bolton was "preparing" for the possibility he could be called to testify in the trial.

- BBC