US Secret Service Director Kimberly Cheatle has rebuffed bipartisan calls to resign for security failures that allowed a would-be assassin to wound Donald Trump in a heating hearing.

The US House of Representatives Oversight Committee’s Republican chair James Comer and top Democrat Jamie Raskin – who are normally bitterly divided on most issues – each called on Cheatle to step down after Monday’s exchange.

“This committee is not known for its model of bipartisanship, and I think today we came together unanimously in our disappointment,” Comer told Cheatle.

“We don’t have that confidence that you can lead.”

Raskin said Cheatle had “lost the confidence of Congress at a very urgent and tender moment in the history of the country, and we very quickly need to move beyond this”.

“I don’t want to add to the director’s terrible, horrible, no good, very bad day, but I will be joining the chairman in calling for the resignation of the director just because I think that this relationship is irretrievable at this point,” he said.

During more than 4½ hours of often contentious proceedings, Cheatle called the July 13 shooting “the most significant operational failure at the Secret Service in decades”, comparing the breakdown to the 1981 attempted assassination of then-president Ronald Reagan.

But she repeatedly rebuffed calls to step down, saying at one point: “I think that I am the best person to lead the Secret Service at this time.”

Monday’s hearing marked the first round of congressional oversight of the attempted assassination of Trump at an outdoor campaign rally in Butler, Pennsylvania.

Trump was wounded in the ear, one rally attendee was killed and another injured.

The suspected shooter, 20-year-old nursing home aide Thomas Crooks, was killed by law enforcement. It is still not clear what his motive was for the shooting.

On Wednesday, FBI director Christopher Wray will appear before the House Judiciary Committee.

House Speaker Mike Johnson is also due to unveil a bipartisan task force to serve as a nexus point for House investigations.

In the face of Republican claims that the Secret Service denied resources to protect Trump, Cheatle said security for the former US president had grown ahead of the shooting.

“The level of security provided for the former president increased well before the campaign and has been steadily increasing as threats evolve,” Cheatle said.

She added that the Secret Service provided the security sought by the Trump campaign for the rally.

But Cheatle repeatedly declined to answer questions from openly frustrated Republicans and Democrats about the security perimeter, what officials knew of potential threats and the decisions they made as well as the suspicious behaviour of the suspect.

“Tell us what went wrong,” Republican representative Pete Sessions said.

“Tell us, and don’t try and play a shell game with us.”

Other Republicans were more bruising, with Nancy Mace telling Cheatle she was “full of s–t”, and Lisa McClain asking if Cheatle Alzheimer’s disease or dementia because she could not recall all the details.

Cheatle said she wanted to provide factual information but cited multiple ongoing investigations, including an internal probe due to be completed within 60 days.

But her answer to one question – “I have a timeline that does not have specifics” – drew exasperated laughter from lawmakers.

Representatives from both parties rejected the idea of a 60-day delay and accused Cheatle of stonewalling Congress.

“The notion of a report coming out in 60 days when the threat environment is so high in the United States, irrespective of party is not acceptable,” Democratic representative Alexandria Ocasio-Cortez said.

-with AAP