A committee investigating the 2021 attack on the US Capitol has voted to subpoena Donald Trump to give evidence.

The panel voted unanimously to compel the former president to appear.

“We must seek the testimony under oath of January 6th’s central player,” said Republican Liz Cheney, the committee’s vice chairwoman.

Mr Trump is almost certain to fight the subpoena and decline to testify.

In the committee’s 10th public session, just weeks before the congressional mid-term elections, the panel summed up Mr Trump’s “staggering betrayal” of his oath of office.

That was how Democratic chairman Bennie Thompson put it, describing Mr Trump’s unprecedented attempt to stop Congress from certifying Democrat Joe Biden’s victory.

With vivid new details and evidence, including from the former president’s Cabinet secretaries and US Secret Service, the panel showed Mr Trump was told repeatedly by those around him that the election was over yet he still orchestrated the far-reaching effort to stop Mr Biden from taking office. Several former aides testified that Mr Trump said privately that he knew he had lost to Mr Biden.

In other striking new video, the panel showed previously unseen footage of congressional leaders phoning officials for help during the Capitol siege.

House Speaker Nancy Pelosi and Senate Democratic Leader Chuck Schumer can be seen talking to governors in neighbouring Virginia and Maryland. Later it shows Senate Republican Leader Mitch McConnell and others in the party asking Mr Trump’s acting attorney general for help.

“They’re breaking the law in many different ways — quite frankly at the instigation of the president of the United States,” Ms Pelosi is heard saying at one point. “Do you believe this?” she exclaims.

The video comes from a documentary being produced by Ms Pelosi’s daughter, sources said.

Earlier, in never-before-seen Secret Service messages, the panel produced evidence that extremist groups provided the muscle in the fight for Mr Trump’s presidency, planning weeks before the attack to send a violent force to Washington.

The Secret Service warned in a December 26 2020, email of a tip that members of the right-wing Proud Boys planned to outnumber the police in a march in Washington on January 6.

“It felt like the calm before the storm,” one Secret Service agent wrote in a group chat.

The House panel warned that the insurrection at the Capitol was not an isolated incident but a warning of the fragility of the nation’s democracy in the post-Trump era.

“None of this is normal or acceptable or lawful in a republic,” Ms Cheney declared.

“There is no defence that Donald Trump was duped or irrational. No president can defy the rule of law and act this way in a constitutional republic, period.”

Statements from Mr Thompson and Ms Cheney were laden with language frequently seen in criminal indictments. Both lawmakers described Mr Trump as “substantially” involved in the events of January 6. Ms Cheney said Mr Trump had acted in a “premeditated” way.

To illustrate what it said were “purposeful lies,” the committee juxtaposed repeated instances in which top administration officials recounted telling Mr Trump the actual facts with clips of him repeating the exact opposite at his pre-riot rally at the Ellipse on January 6.

The committee may well make a decision on whether to make a criminal referral to the Justice Department.

Thursday’s hearing opened at a mostly empty Capitol complex, with most lawmakers at home campaigning for re-election. Several people who were among the thousands around the Capitol on January 6 are now running for congressional office, some with Mr Trump’s backing. Police officers who fought the mob filled the hearing room’s front row.

In one recorded interview, former White House official Alyssa Farah Griffin said that Mr Trump looked up at a television and said: “Can you believe I lost to this (expletive) guy?”