Lawmakers probing the 2021 attack on the US Capitol voted on Thursday to subpoena former president Donald Trump to testify on his involvement in the violence, in a major escalation of its sprawling inquiry weeks before it is due to wind up.

In what was expected to be its final hearing before the midterm elections, the House panel of seven Democrats and two Republicans agreed unanimously to compel Trump's appearance before investigators.

"We need to be fair and thorough and gain a full context for the evidence we've obtained. But the need for this committee to hear from Donald Trump goes beyond our fact-finding," said Bennie Thompson, the chairman of the committee.

"This is a question about accountability to the American people. He must be accountable. He is required to answer for his actions."

Subpoenas from the panel have proved difficult to enforce, with former White House aide Steve Bannon the only target convicted of contempt of Congress for refusing to comply.

Trump is notorious for his ability to run down the clock on congressional investigations and legal action, and it remains highly unlikely that he would agree to give evidence.

Any subpoena would expire in any case with the new congressional term in January. The House of Representatives is expected to be flipped in November elections to the Republicans, who plan immediately to end the investigation.

But the move marks an aggressive escalation of the probe, which has issued more than 100 subpoenas and interviewed more than 1,000 people since its inception in 2021.

While no sitting president has ever been forced to testify before Congress, lawmakers have summoned former presidents to discuss their conduct in office.

There was no immediate response from Trump. But if he refuses to comply, the full House can hold him in criminal contempt in a vote recommending him for prosecution.

The vote came as a spectacular coda to an already stunning hearing in which the committee offered fresh evidence that Trump had planned to declare victory in the 2020 election – regardless of the outcome.

Trump had a "premeditated plan" formulated months before the vote to claim he had won on election night, whatever the vote tally showed, panel member Zoe Lofgren told the hearing, citing evidence gathered by the committee.

Across eight hearings in the summer the panel unveiled reams of evidence on the former president's involvement in a labyrinthine series of connected schemes to overturn the election.

"The vast weight of evidence presented so far has shown us that the central cause of January 6 was one man – Donald Trump – whom many others followed," said committee deputy chair Liz Cheney.

The committee also pressed its position that Trump – who continues to be a wellspring of disinformation about the 2020 presidential election – remains a "clear and present" threat to democracy. (AFP)