Over the years, whenever the United States Secret Service’s lack of resources has been an issue, agents invoke a tongue-in-cheek motto: We do more with less.

It’s a sentiment that might no longer apply.

An assassination attempt on former President Donald J. Trump at a campaign rally on July 13 in Butler, Pa., has exposed weaknesses in the Secret Service’s make-do approach to security for the country’s highest elected officials, their families, and visiting foreign leaders.

Kimberly A. Cheatle, the then-Secret Service director, told a congressional committee on Monday that the shooting — which left the former president’s ear bloodied and injured three other spectators, one fatally — “is the most significant operational failure at the Secret Service in decades.” She resigned the day after the hearing.

The agency’s security lapses in Butler have spurred numerous investigations and prompted debate as to whether the Secret Service, which is operating with its largest-ever budget, has adequate resources to fulfill its critical mandate and is using its funds in the most effective manner.

“The Secret Service is conducting a top-to-bottom review of all of our operational security policies and procedures to see where we may need to request additional resources to bolster our protective mission,” a spokesman for the Secret Service said in a statement to The New York Times.

Interviews with current and former Secret Service agents reveal an agency that wears down its employees, risking their sharpness. They portray an organization that spends so much money on physical protection that there are few funds left to stay abreast of the latest law enforcement training and technologies.