The US Senate voted on Saturday to allow witnesses at the impeachment trial of Donald Trump, a surprise move likely to extend the proceedings, even as the top Republican senator said he would vote to acquit the former president of inciting the deadly January 6 assault on the Capitol.

Republican Minority Leader Mitch McConnell's decision to acquit means Trump is unlikely to be convicted by the Senate of inciting an insurrection by his supporters.

The second impeachment trial of the 74-year-old former Republican president had been expected to conclude with final arguments and a verdict on Saturday.

But in a surprise move that could potentially prolong the trial for an undetermined time, the lead House impeachment manager, Jamie Raskin, said he wanted to call a Republican lawmaker as a witness.

"We believe we've proven our case," Raskin said, but he wanted Representative Jamie Herrera Beutler to testify because she can provide an "additional critical piece of corroborating evidence".

Raskin's demand prompted a threat by Trump's defense lawyers to call witnesses of their own, including Vice President Kamala Harris, Nancy Pelosi, the Democratic speaker of the House of Representatives, and others.

The Senate voted 55-45 to allow witnesses, with five Republicans joining their Democratic colleagues.

The move sparked confusion on the Senate floor and it was not immediately clear how many — if any — witnesses would eventually be called to testify.

Raskin said he wanted to call Herrera Beutler, a Republican from Washington state, to testify after she released a statement about the events of January 6.

Herrera Beutler was one of 10 Republican lawmakers who voted to impeach Trump in the House of Representatives.

In her statement, she said Republican House Minority Leader Kevin McCarthy had called Trump while the attack was ongoing and implored him to call off the rioters.

"When McCarthy finally reached the president on January 6 and asked him to publicly and forcefully call off the riot, the president initially repeated the falsehood that it was Antifa that had breached the Capitol,” Herrera Beutler said.

"McCarthy refuted that and told the president that these were Trump supporters,” the congresswoman said.

"That’s when, according to McCarthy, the president said: ‘Well, Kevin, I guess these people are more upset about the election than you are,’” she said.

‘A close call’ 

Trump was impeached by the Democratic-controlled House on January 13 for inciting the attack on the US Capitol by his supporters, who were seeking to block congressional certification of Democrat Joe Biden’s November 3 election victory.

Texas Senator Ted Cruz, a Trump ally, told reporters he did not think witnesses were necessary.

"My view remains we don’t need witnesses,” Cruz told reporters. "This is a political theatre.”

He said Democrats did not have the 17 Republican votes needed to convict Trump.

The other Republican senator from Texas, John Cornyn, said calling witnesses could cause the trial to "drag on indefinitely”.

"Kangaroo court anyone?” Cornyn tweeted. "No end in sight.”

A conviction in the 100-member Senate — which is evenly split between Democrats and Republicans — would require a two-thirds majority and appears unlikely in any case after McConnell said he would vote to acquit.

"While a close call, I am persuaded that impeachments are a tool primarily of removal and therefore we lack jurisdiction,” McConnell said in an email to his Republican colleagues.

"The Constitution makes perfectly clear that presidential criminal misconduct while in office can be prosecuted after the president has left office,” he said. "Given these conclusions, I will vote to acquit.”

Trump’s defense lawyers argued on Friday that the former president bears no responsibility for the attack on Congress and wrapped up their presentation in just three hours.

This followed two days of evidence from Democratic impeachment managers centered around harrowing video footage of the mob assault on the Capitol.

Trump’s defense lawyers called the impeachment unconstitutional and an "act of political vengeance”.

They argued that Trump’s rally speech near the White House that preceded the January 6 attack, when he told supporters to "fight”, was merely rhetorical.

Seeking to turn the table on the Democrats’ powerful use of video evidence, defence lawyers played their own compilations showing Democratic lawmakers at different times using the word "fight”.

House impeachment managers charge that after losing to Biden, Trump deliberately stoked tension with a campaign of lies claiming there had been mass voter fraud.

On January 6 he staged a fiery rally near the White House, calling on the crowd to march on Congress, which was in the process of certifying Biden’s victory.

The mob then stormed the Capitol, disrupting the certification. Five people, including a police officer and a woman shot during the unrest, died in the mayhem.