WASHINGTON — As the mob pushed its way through the Capitol’s Crypt on Jan. 6, Officer James Blassingame was slammed back against a stone column and nearly overrun. He saw hate in the eyes of the rioters, hoisting Trump flags and “Make America Great Again” hats, as they urinated on the walls where American icons have served and called him racist slurs.

“Legitimately, I did not think I was going to make it home,” Officer Blassingame, 40, and a 17-year veteran of the Capitol Police force, said in a recent interview in which he described his experience in detail.

He did survive, but the horrors of Jan. 6, when supporters of President Donald J. Trump violently breached the Capitol, had a profound effect on Officer Blassingame. He was injured in the head and back. He said he avoids certain hallways at the Capitol, struggles with feelings of guilt and routinely has flashbacks of fighting off the mob.

And his personal trauma mirrors a broader crisis within the U.S. Capitol Police, which is badly damaged, demoralized and depleted six months after the attack.

“We have people retiring like crazy; we have people quitting,” said Officer Blassingame, who filed a lawsuit with another officer against Mr. Trump for damages for their physical and emotional injuries. “I have friends of mine who have literally come in and quit. They don’t even have jobs.”

Half a year after the Capitol riot, the 2,000-member police force charged with protecting Congress finds itself at perhaps its biggest crossroads in its nearly two-century existence. Its work force is traumatized and overworked as its ranks have been hollowed out by a flood of departures. The agency is facing possible furloughs as it teeters on the brink of running out of funding as overtime costs outpace its budget for salaries. It has been besieged by criticism by members of both parties for the stunning security failures that allowed the assault to occur. And on top of it all, its officers have become the target of conspiracy theories by Republican lawmakers who, following Mr. Trump’s lead, have suggested that a Capitol Police officer premeditated the killing of Ashli Babbitt, a rioter who was shot steps away from the door to the House chamber.

“There’s a lot of exhaustion,” says Representative Tim Ryan, Democrat of Ohio, the chairman of a subcommittee that oversees the Capitol Police. “They’re tired. I think they feel betrayed and unappreciated. And they were betrayed on Jan. 6, primarily by the leadership. They’re embarrassed about how it went down.”

The agency says more than 70 officers have retired or resigned since the Jan. 6 attack, which cost the lives of two members of the force who battled the rioters: Officer Brian D. Sicknick, who died from a stroke, and Officer Howard Liebengood, who took his own life. Officials say the departure rate is slightly higher than normal, but Gus Papathanasiou, the chairman of the Capitol Police union, said he believed the rate was far worse than was being disclosed.

“This year has been the worst I’ve seen since I’ve been here,” said Mr. Papathanasiou, who has served on the force for nearly 20 years.

More than 80 Capitol Police officers reported being seriously injured at the riot, though that number, too, is most likely higher, because many have opted not to report their injuries, according to Capt. Carneysha Mendoza, the commander of the Civil Disturbance Unit, who had facial burns that lasted weeks.

During the mayhem on Jan. 6, officers lacking helmets sustained brain injuries, cracked ribs and shattered spinal discs. One was stabbed with a metal fence stake. Another lost the tip of his right index finger. Still more were smashed in the head with baseball bats and flag poles. Dozens, if not hundreds, of officers are expected to experience post-traumatic stress disorder, experts say.

Only weeks later, a third officer, Billy Evans, was killed when a man crashed his car into the barricade he was guarding in the driveway of the Capitol.

In June, a 127-page joint report by two Senate committees presented a damning portrait of the agency’s preparations and response at multiple levels. Its leaders did not take seriously grave and specific threats of violence at the Capitol and against lawmakers, it found, and the force lacked the training and preparation to respond effectively when those threats materialized.

Documents obtained by The Times from a public records request filed by the group Property of the People, show that the F.B.I. warned the Capitol Police and the District of Columbia Metropolitan Police that extremist groups would be attending the Jan. 6 protests and “planned to use specific radio frequencies for their communication.” The District’s emergency communications office then programmed some hand-held radios to those frequencies and gave them to the Capitol Police and the Metropolitan Police Department to use for monitoring, the documents show.

The dissemination of the radios, which was confirmed by the Capitol Police, is more evidence that law enforcement agencies were well aware of the involvement of organized extremists in the “stop the steal” protests and the threats against the Capitol.

Investigators have also looked into allegations that some Capitol Police officers were complicit in the riot. As part of the sprawling discovery process in the criminal cases stemming from the attack, the Justice Department has agreed to give defense lawyers copies of reports into those accusations, according to court papers filed on Monday.

Increasingly, there are calls for a near-complete overhaul of the agency. Top members of Congress say the acting Capitol Police chief, Yogananda D. Pittman, cannot continue to lead the agency, after the union representing officers voted shortly after the riot that it had “no confidence” in her and six other senior officials in the department.

But efforts to turn the page have been rocky. After the attack, Steven A. Sund, the Capitol Police chief, resigned along with the top House and Senate security officials, a move that left raw feelings on the force among some who remain deeply loyal to Mr. Sund.

Capitol Police leaders say the agency is making major changes. They have instituted better training that involves holding joint sessions with the National Guard and sending officers to learn from agencies in Seattle and Virginia Beach. The force plans to purchase more protective equipment and surveillance technology, funded in part by a loan from the Department of Defense. It will begin opening field offices around the country, starting in California and Florida, to help monitor and quickly investigate threats against members of Congress wherever they occur.

And the agency has increased its mental health services since the riot, including bringing in police from other agencies for peer counseling and adding two new emotional support dogs, named Lila and Filip.

But other proposed upgrades cannot be funded without additional support from Congress, which is locked in a stalemate over the money amid Republican opposition to a $1.9 billion emergency security bill, including concerns about militarizing the Capitol with a quick reaction force of National Guard troops.

Senator Patrick J. Leahy, Democrat of Vermont and the chairman of the Appropriations Committee, has warned that the Capitol Police force will run out of money by August if the Senate fails to pass the security funding bill that the House approved in May over unanimous opposition from Republicans.

Already, Mr. Leahy said, the Capitol Police has delayed purchases of “critical equipment,” such as helmets and protective gear, because of the looming funding lapse. A wellness program that was intended to further address mental health concerns in the agency has been put on a “back burner,” he said.

The situation, he said, amounts to Congress turning its back “on those who fought, bled and died on that day.”

Senator Richard C. Shelby of Alabama, the top Republican on the appropriations panel, said his party had proposed a much narrower bill that would provide money for the Capitol Police while lawmakers studied what security upgrades might be needed.

That roughly $630 million proposal, a draft of which was obtained by The Times, would provide about $97 million for the Capitol Police, but did not include money for security improvements at the complex.

“We should pass now what we all agree on,” Mr. Shelby said in a statement to The Times on Friday. “The Capitol Police and National Guard are running out of money, the clock is ticking, and we need to take care of them.”

The funding impasse compounds a mounting sense of betrayal that many Capitol Police officers say they have experienced as some Republicans loyal to Mr. Trump, whose lies of a stolen election egged on the mob on Jan. 6, have worked to deny, downplay or justify the attack. Last month, 21 House Republicans voted against awarding Congressional Gold Medals to Capitol Police officers who responded to the riot. Senate Republicans blocked the formation of an independent bipartisan commission to investigate what happened, even after officers on duty that day and the family of Officer Sicknick personally pleaded with them to allow it to go forward.

In recent weeks, some Republicans have spread conspiracy theories, such as the baseless claim that the F.B.I. was secretly behind the Capitol siege. And some have latched onto the shooting of Ms. Babbitt, who was part of a mob that broke through a glass door only steps away from lawmakers when she was fatally shot, to suggest that the Capitol Police were targeting Mr. Trump’s supporters.

Some lawmakers have credited the Capitol Police officer who shot Ms. Babbitt with saving their lives, but one House Republican, Representative Paul Gosar of Arizona — who has a history of associating with extremists and white nationalists — accused the officer of “lying in wait” to carry out an “execution.”

Mr. Trump has recently begun questioning the shooting and why the name of the officer who shot Ms. Babbitt has not been released, a question raised by a growing number of Republicans, including Representative Marjorie Taylor Greene of Georgia, who has said that lawmakers should “demand justice” for Ms. Babbitt.

The Capitol Police, like Congress, is not subject to public records requests, and lawyers representing the officer say he has received death threats. The Justice Department in April closed its investigation of the shooting and declined to pursue criminal charges against the officer, though Ms. Babbitt’s husband has sued to force the release of investigative files related to the shooting and her family has threatened to seek damages from the Capitol Police for her killing.

To officers who responded to the mob on Jan. 6, the reaction by Republicans loyal to Mr. Trump only adds to an already untenable situation they are facing inside a broken department.

“We go to work every day to protect Congress, and these people won’t even have our back,” Officer Blassingame said. “The officers did our job — no member of Congress was injured on that day. For them to not have our back, it’s extremely disheartening.”

Charlie Savage and Alan Feuer contributed reporting.