US Secret Service Director Kimberly Cheatle resigned on Tuesday, a day after acknowledging that the agency failed in its mission to prevent an assassination attempt against former President Donald Trump.

Cheatle was facing bipartisan calls to step down after a 20-year-old gunman wounded the  Republican presidential candidate at a July 13 campaign rally in Butler, Pennsylvania.

"It is overdue, she should have done this at least a week ago," Mike Johnson, the Republican Speaker of the House of Representatives, told reporters. "I'm happy to see that she has heeded the call of both Republicans and Democrats."

Cheatle appeared before a congressional committee on Monday and said the attack on Trump, who was slightly wounded in his right ear, represented a failure by the Secret Service.

She called it "the most significant operational failure of the Secret Service in decades."

Both Republicans and Democrats called on Cheatle to resign. She drew the ire of lawmakers from both parties by refusing to provide specific details about the attack, citing the existence of multiple active investigations.

The gunman opened fire on Trump with an AR-style assault rifle just minutes after he began speaking at the campaign event.

Perched on the roof of a nearby building, he was shot dead by a Secret Service sniper less than 30 seconds after firing the first of eight shots.

Investigators have concluded that the young man, who lived in a town about 50 miles (80 kilometers) from Butler, acted alone and could not identify any strong ideological or political leanings.

Two rally attendees were seriously wounded and a 50-year-old Pennsylvania firefighter, Corey Comperatore, was shot dead.

Trump's former physician said over the weekend that the Republican candidate sustained a two-centimeter (almost one inch) gunshot wound on his right ear.