“We failed,” Cheatle said in testimony before the House of Representatives Oversight Committee.

“The assassination attempt on former president Donald Trump on July 13th is the most significant operational failure at the Secret Service in decades.”

Republican and Democratic lawmakers called on her to resign, calls that she rebuffed, saying at one point, “I think that I am the best person to lead the Secret Service at this time.”

In the face of Republican claims that the Secret Service denied resources to protect Trump, Cheatle said security for the former 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.

She declined to answer specific questions about the day’s security plan from openly frustrated Republicans and Democrats, saying the matter was being investigated internally.

Monday’s hearing marked the first round of congressional oversight of the attempted assassination.

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.

Republican House Oversight Committee James Comer called for Cheatle to step down.

“It is my firm belief, Director Cheatle, that you should resign,” the Kentucky Republican told her. “The Secret Service has thousands of employees and a significant budget. But it has now become the face of incompetence.”

Democratic Representative Ro Khanna also called for her to resign.

“If you have an assassination attempt on a president, former president or a candidate, you need to resign,” the California Democrat said. “You cannot go leading the Secret Service agency, when there is an assassination attempt on a presidential candidate.”

Another Democrat, Representative Gerry Connolly, said the shooting highlighted the increasingly polarised state of a nation experiencing heightened political tensions.

But he expressed exasperation when Cheatle declined to say her job was complicated by the easy availability of guns, especially assault rifles like the one used in the Trump shooting.

“And you wonder why we might have a lack of confidence in your continued ability to direct this agency,” Connolly said.

The shooting at an outdoor campaign rally in Butler, Pennsylvania, wounded Trump in the ear, killed one rally attendee and injured another.

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

The House Judiciary Committee said last week that it had evidence the Secret Service was not properly resourced for Trump’s rally, because of staffing shortages created by a rival campaign event in Pittsburgh with Jill Biden and a Nato summit held days before in Washington.

Cheatle told lawmakers that the agency protected 36 individuals daily, as well as world leaders who visited the United States.

President Joe Biden on Sunday ended his floundering re-election bid, endorsing Vice President Kamala Harris to succeed him as candidate. He vowed to serve through the end of his term on January 20, 2025.