A Secret Service sniper claims that another assassination attempt against a presidential candidate seems inevitable before election day because the attack on former President Donald Trump exposed the weakness in Secret Service security, according to a scathing letter circulating within the agency.

“This agency NEEDS to change, if not now, WHEN? The NEXT assassination attempt in 30 days?” the letter, first published by RealClearPolitics, read.

“We all SHOULD expect another attempt to happen before November. We’ve exposed our inability to protect our leaders due to our leadership.”

Law enforcement sources verified the authenticity of the letter to The Post. It was sent Monday around to the entirety of the Secret Service Uniformed Division — the agency’s police force that secures the White House.

The sniper demanded the resignation of high level supervisors, whom the agent accused of failing rank-and-file Secret Service staff.

“Sadly we have fallen short for YEARS. We just got lucky and looked good doing it. I have conveyed these thoughts to not only supervisors … Only to be brushed off as those with less experience somehow knew more than me,” the letter read.

“Secret Service SUPERVISORS ‘knew better’ and the foot soldiers working, made the best of a BAD situation.”

It went on to say that the reputation of the Secret Service and all its agents had been marred by the failures of July 13, which resulted in Trump being shot in the ear and a hero firefighter behind him being killed. The sniper said the day was “a stain I will never be able to cleanse.”

The identity of the counter-sniper who wrote the letter is unclear. They described themselves as a veteran of the Marine Corps, and a member of Secret Service’s counter sniper team for more than 20 years.

Since the disastrous assassination attempt, heavy scrutiny has fallen on the Secret Service’s handling of security for former presidents.

Law enforcement sources previously told The Post a full 30 seconds elapsed between when local police confronted gunman Thomas Crooks and the first shots he fired at Trump — prompting questions as to why Trump was allowed to remain onstage.

The acting Secret Service director said Tuesday that agents were never warned Crooks was on the roof with a rifle.

Meanwhile, newly revealed footage from the site of the shooting showed local cops encircling the building he shot from for more than two minutes before the attack, reinforcing that baffling question.

Local police were in charge of the grounds where Crooks fired from, but the Secret Service was ultimately in charge of coordinating the day’s security.