Former US president Donald Trump was acquitted Saturday for the second time in a year in an impeachment trial in the Senate.

The vote was 57-43 in favor of conviction, but 67 votes, a two-thirds majority, are required for a conviction in the Senate following a House impeachment. Seven members of Trump's Republican Party voted against him.

The single impeachment article was for "incitement of insurrection" stemming from Trump's speech on Jan 6, when his supporters stormed the Capitol building while the Electoral College vote certifying Joe Biden's victory in the Nov 3 presidential election was taking place.

Senate Minority Leader Mitch McConnell of Kentucky, who had been harshly critical of Trump before the trial and afterward on Saturday, still voted to acquit.

"There is no question that President Trump is practically and morally responsible for provoking the events of the day," he said. "The people who stormed this building believed they were acting on the wishes and instructions of their president."

The Republican senators who voted to find Trump guilty were Richard Burr of North Carolina, Bill Cassidy of Louisiana, Susan Collins of Maine, Lisa Murkowski of Alaska, Mitt Romney of Utah, Ben Sasse of Nebraska and Patrick Toomey of Pennsylvania.

"This has been yet another phase of the greatest witch hunt in the history of our country," Trump said in a statement after the Senate vote. "No president has ever gone through anything like it, and it continues because our opponents cannot forget the almost 75 million people, the highest number ever for a sitting president, who voted for us just a few short months ago."

"Just look at what Republicans have been forced to defend," Senate Majority Leader Chuck Schumer said. "Look at what Republicans have chosen to forgive."

There was some trial drama on Saturday morning when the Senate voted 55-45 to call witnesses, namely Republican Congresswoman Jaime Herrera Beutler of Washington state. She had recounted a conversation with House Minority Leader Kevin McCarthy of California, who told her Trump didn't respond to a plea to call off the Capitol rioters and initially blamed the siege on antifa.

"Well, Kevin, I guess these people are more upset about the election than you are," Trump said, according to a statement published Friday on Herrera Beutler's official website.

The situation was resolved when her statement about the call was read aloud into the record for senators to consider as evidence. As part of the deal, Democrats did not proceed with deposition of the congresswoman, and Republicans did not call their own witnesses.

Also included was the timing of a call by Senator Mike Lee, a Utah Republican, with Trump around the time former vice president Mike Pence was evacuated, minutes after Trump criticized Pence in a tweet.

The Saturday vote capped a dramatic week, mostly centered on videos played earlier by both sides. The Democrats started with video of chaotic scenes of shouting Trump supporters barging into the Capitol while members of Congress were evacuated to safety. Five people died in connection to that day's violence.

On Friday, Trump's defense team countered with its own montages, showing numerous Democratic politicians making references to "fighting", as Trump's exhortation to his supporters to "fight like hell" had been stressed by House impeachment managers.

Before the trial, much of the Republican defense argument centered on the constitutionality of whether a former president could be impeached.

Supreme Court Chief Justice John Roberts declined to preside at the trial, as there was uncertainty whether it was appropriate under the Constitution for him to participate in a proceeding against someone who was no longer president. Instead, Senate President pro tempore Patrick Leahy, Democrat from Vermont, oversaw the proceedings.

But the constitutional strategy was less evident on Friday as Trump's lawyers went on the offensive, blaming Democrats for not speaking out against the violence that took place in US cities across the summer following the May 25 death of George Floyd in Minneapolis at the hands of police officers.

For all the commentary the second impeachment trial generated, there seemed to be an unspoken bipartisan consensus it should be brief.

Some Democrats were concerned a long show trial would delay Biden's agenda, particularly in dealing with the coronavirus pandemic, and saw no political benefit from a second acquittal of Trump.

And Republicans had to consider if 17 or more of them had sided with Democrats and voted to convict Trump, the GOP could have faced the prospect of Trump, 74, or his supporters forming a breakaway party, even though he has said he would not.