WASHINGTON — Leaders of the FBI and the Justice Department made an urgent appeal to White House Chief of Staff John F. Kelly last winter to side with them against Republicans in Congress who were pressing for information about the Russia investigation that would compromise confidential sources.

Kelly seemed to agree. But not long after Rod Rosenstein, the deputy attorney general, and Christopher A. Wray, the FBI director, returned to their offices, Kelly called back and reversed himself, according to a former law enforcement official.

They would have to hand over the information after all.

What changed, Kelly said, was the president.

For more than a year, President Trump has been at war with law enforcement agencies that answer to him, interjecting himself into an investigation in which he himself is a subject.

And he has escalated the conflict dramatically in recent days by accusing the FBI of placing a “spy’’ inside his 2016 campaign, pressuring the agencies to reveal secret information, and demanding an investigation of his investigators.

The confrontation has no precedent in the modern era and holds great stakes not just for the president but for the relative autonomy for law enforcement investigations established after Watergate.

Trump’s allies argue that he has every right to manage the executive branch and every reason to be outraged at possible misconduct aimed at his campaign. But many law enforcement veterans say he is wreaking untold damage on institutions that form the bulwark of a democratic society.

“To turn on the FBI using this loaded language like ‘spy’ and ‘infiltrate,’ President Trump is trying to poison public opinion against the FBI for his own reasons,’’ said Barbara McQuade, a career federal prosecutor who served as US attorney in Michigan under former president Barack Obama.

“He may be successful, but I worry about the impact his kind of rhetoric has on the public when the FBI is investigating a case of kidnapping or bank robbery and the president has told them they’re not trustworthy,’’ she said.

Since even before taking office, Trump has disparaged intelligence agencies that concluded that Russia sought to influence the election on his behalf, at one point in effect comparing them to Nazis.

He has publicly badgered law enforcement officials to shut down the Russia investigation and instead to open inquiries into his political adversaries. But he went even further last week by effectively ordering an investigation into the actions taken regarding his campaign.

He also summoned Wray and Rosenstein to the White House to insist they turn over details about the FBI informant that he called a “spy’’ and even sent Kelly and a White House lawyer, Emmet T. Flood, to a meeting that Wray and Rosenstein held with congressional leaders about the confidential information.

Although the White House said they did not stay, Democrats called their presence highly inappropriate.

David B. Rivkin Jr., a constitutional lawyer who worked in the White House and the Justice Department under Presidents Ronald Reagan and George Bush, said critics are overreacting. While Trump’s bellicose tweets and direct involvement are unusual, he said, it would not be any different legally had he gone through his White House counsel, Donald McGahn.

“What he did was not only perfectly constitutional but was not unreasonable,’’ said Rivkin. “He perhaps could have done it more elegantly, maybe he should have used McGahn, but he is who he is and we shouldn’t hold it against him. Just because something isn’t elegant doesn’t mean it’s wrong.’’

Rivkin and others said Trump was right to see the use of an informant as an abuse of power, equating it to FBI surveillance of civil rights and antiwar groups in the 1960s.

Jonathan Turley, a law professor at George Washington University, said that while the way Trump went about seeking an investigation was “clearly inappropriate,’’ the emerging facts merited an examination.

“Where I depart from the critics is I think the president has a legitimate point here,’’ he said. “The extent of this investigation directed against an opposition party’s presidential campaign is unprecedented and it does raise legitimate questions.’’

Contrary to Trump’s assertions, no evidence has surfaced that the FBI informant or anyone else was “embedded’’ inside Trump’s campaign or that his work had partisan origins.

The FBI was investigating Russian efforts to influence the US election and had picked up evidence that people around Trump had been in contact with Russians, evidence that has only grown in the nearly two years since then.

Trump has wanted to take on his own government more overtly for months, but had been warned against it by his lawyers. Since bringing on Rudy Giuliani, the former New York mayor, as the face of his legal team, the president has turned more aggressive and is willing to push boundaries of law enforcement practices.

The president’s demand for an investigation into the informant and the release of confidential information stunned Justice Department officials and put Rosenstein in an awkward position. Rather than resist, Rosenstein released a statement agreeing to an inquiry.

Current and former law enforcement officials have mixed opinions on Rosenstein’s handling of the situation. Some applaud him for mollifying a president who has openly mused about firing Mueller. Others say he has weakened the Justice Department’s principles and contributed to Trump’s efforts to undermine Mueller’s credibility.