Her name evokes an outlaw in the American Old West, and Cassidy Hutchinson did not disappoint as the young gunslinger gave it to Donald Trump with both barrels.

Once an ardently loyal footsoldier in the former commander-in-chief's posse, Hutchinson turned sheriff Tuesday as she fired off a volley of allegations without historical parallel against an American president.

In a blockbuster appearance before the House committee investigating last year's attack on the US Capitol, the top former White House aide delivered what opponents hope will be the evidence needed to run Trump out of town.

"This is the smoking gun," Sol Wisenberg, a former deputy to Bill Clinton impeachment investigator Ken Starr, told The New York Times of Hutchinson's testimony.

"There isn't any question this establishes a prima facie case for his criminal culpability on seditious conspiracy charges."

There may have been no "wanted" poster for Trump but Hutchinson painted her own portrait -- of an unhinged president unable to cope with defeat, bidding at any cost to halt the peaceful transfer of power.

Crucially, she offered what critics of the investigation say has so far been lacking -- testimony that Trump not only knew his election fraud claims were false but that he was aware of the potential violence they would cause, and encouraged it.

An erratic leader who often overturned tableware in fits of rage, Trump demanded to be driven to the Capitol to be with the insurrectionist mob after the violence had already broken out, assaulting his Secret Service detail when the order was refused, according to a second-hand anecdote Hutchinson relayed.

'Broken porcelain'

David Greenberg, a journalism and history professor at Rutgers University-New Brunswick, called Hutchinson's testimony "riveting and revelatory."

"Historical accounts of January 6 won't fail to include her vivid descriptions of ketchup oozing down the wall and the broken porcelain plate -- a result of an enraged Trump having hurled his lunch across the room," he told AFP.

"Substantively, it was damning, especially in revealing how ready Trump was to unleash armed protesters on the Capitol."

Hutchinson said Trump knew his supporters were armed, including with Glock pistols and AR-15-style semi-automatic rifles, and didn't care, telling them to march to the Capitol and "fight like hell" anyway.

"Cassidy Hutchinson's testimony was credible, chilling and highly damaging to former president Trump and his allies," Mike Hernandez, a political analyst for Miami-based Telemundo 51, told AFP.

Although it won't move the needle much among Trump's ultra-loyal base, according to Hernandez, it could convince sufficient numbers of Republicans that he should not be the nominee in 2024.

The march on the Capitol, as lawmakers were certifying Trump's defeat to Joe Biden, had been characterized as a spontaneous idea but Trump's plan to join his supporters was premeditated, according to Hutchinson's testimony.

She recounted that White House counsel Pat Cipollone told her to ensure that the motorcade did not make the trip because they'd get "charged with every crime imaginable."

'She's seen everything'

The Secret Service is reportedly prepared to deny the assault anecdote under oath, and some Republicans have dismissed parts of Hutchinson's testimony as "hearsay," but few have pushed back against the core allegations.

The first live witness from the West Wing to testify, Hutchinson was something of an unknown quantity to the public.

What seemed clear was that her loyalty to Trump had never been in question. The ex-president himself said after her testimony that she was a "big Trump fan long after January 6."

The New Jersey native had honed her interest in the business of government, studying for a political science degree in Virginia before interning for senior Republicans in Congress.

She took a role at the White House's legislative affairs office and was promoted to be the principal aide to Meadows in March 2020, allocated a desk just a few steps from the Oval Office.

Working daily with lawmakers and White House officials at all levels, she was "in a position to know a great deal about the happenings in the White House," committee chair Liz Cheney said in her introduction.

Trump took to his Truth Social app, to fire off a 12-post rant dismissing Hutchinson as low-level and calling her a "total phony."

But several of her former colleagues vouched for her, arguing she was perfectly placed to report on activity at the very top.

Alyssa Farah Griffin, the White House director of strategic communications in Trump's final year in office, told CNN that Hutchinson was on a first-name basis with most members of congressional leadership.

"She would text with them. So she's seen everything," Farah Griffin said. "She's been in so many rooms."