From the moment President Trump was re-elected, he has muscled his way into the driver’s seat at the Justice Department, leaving tread marks on its time-honored independence from the White House by demanding, and sometimes directing, investigations into his enemies.

But his tight grip on the steering wheel has often threatened to lead the department off a cliff.

Last week offered a particularly vivid portrait of the consequences of Mr. Trump’s meddling, which, in the span of only a few days, imperiled one criminal case, set the stage for sanctions in a second and gummed up the chances that his pick for attorney general will be confirmed by the Senate.

Mr. Trump has long believed that he is his own best adviser and that, as the leader of the executive branch, he enjoys complete control over the agencies beneath him. But there is a reason the department in charge of federal law enforcement has traditionally stood outside of the president’s personal purview, said Norman L. Eisen, a lawyer involved in some of the recent cases who, earlier in his career, worked on the Obama administration’s “no contact” policy with the Justice Department.

“When presidents meddle in the department,” Mr. Eisen said, “it tends to disrupt its normal functioning.”

The problems last week began on Wednesday, when lawyers for James B. Comey, the former F.B.I. director, filed court papers revealing that Secret Service agents who questioned their client about a photograph he posted showing seashells on a beach arranged to read “86 47” were ordered to quickly send a summary of the interview to Mr. Trump on Air Force One. The interview was “a hot topic on the plane,” and the president was “very interested,” the filing said. In fact, it added, Mr. Trump wanted to use the agents’ notes as fodder at a coming media appearance even though they presumably contained sensitive and unverified investigative information.

The request for the notes was a shocking breach of the distance that most presidents have kept from the day-to-day oversight of criminal investigations and indicated the intensity of Mr. Trump’s personal involvement in the pursuit of Mr. Comey, one of his most reviled political adversaries.

But his insistence on interfering with the case could come back to haunt him. After all, Mr. Comey’s lawyers pointed to the episode as evidence that the Justice Department had indicted him — on charges of making threats against the president — only as an act of retribution. If that claim is proven true, it would result in the case being dismissed.

Mr. Trump’s intrusive instincts also played a role last week in the demise of another criminal case, one that accused the former Olympic canoeist David Hearn with vandalizing one of the president’s pet projects, the Lincoln Memorial Reflecting Pool.

When Jeanine Pirro, the U.S. attorney in Washington, announced the indictment of Mr. Hearn last month, she made it clear that the charges had been brought, at least in part, because Mr. Trump wanted them.

“This is not only a priority for the president,” she told reporters, “but for myself.”

On Friday night, however, the case fell apart in a spectacular fashion. Ms. Pirro, in a court filing, suddenly acknowledged that new information from Mr. Trump’s Interior Department had made her realize that shoddy construction, not anything that Mr. Hearn had done, caused the problems with the pool’s lining.

But even that confession did not stop Mr. Trump from doubling down on the vandalism narrative. On Saturday morning, he posted a message on social media saying that he disagreed entirely with Ms. Pirro, a veteran prosecutor and a friend of his for more than 40 years.

“I don’t know what she was thinking?” Mr. Trump wrote.

While Mr. Hearn’s lawyers have not disclosed what they might do next, the case’s collapse gives them an opening to argue that the president’s stubborn statements about the matter suggest the prosecution was vexatious, frivolous or brought in bad faith.

Should a move like that succeed, it would add to the roster of Ms. Pirro’s failures to go after Mr. Trump’s enemies — a list that already includes six Democratic lawmakers accused of sedition by the president after they posted a video reminding military personnel of their obligations to ignore illegal orders. It would also allow Mr. Hearn to recoup legal fees for the cast of lawyers he hired to defend himself.

The president’s inability to keep his hands to himself also affected another important bit of business at the Justice Department: the effort to confirm Todd Blanche, Mr. Trump’s former personal lawyer, as attorney general.

As the week came to an end, Mr. Blanche seemed well on his way to removing the main hurdle to his appointment, calming the concerns expressed by two Republican senators — John Cornyn of Texas and Thom Tillis of North Carolina. The lawmakers have repeatedly voiced unhappiness with the $1.8 billion compensation fund and the extraordinary personal tax protections that emerged this spring from Mr. Trump’s deal to drop his lawsuit against the Internal Revenue Service.

But as the weekend neared, the president seemed to throw Mr. Blanche under the bus that he himself was driving, issuing a series of confusing and contradictory messages.

On Friday, he appeared to step back from his earlier threat to withdraw Mr. Blanche’s nomination and repeated his support for creating the fund, which Mr. Blanche had already said was dead. On Saturday, Mr. Trump again asserted that he would revive the fund, which could be used to compensate rioters who stormed the Capitol on Jan. 6, 2021, if the Senate did not confirm Mr. Blanche.

Mr. Tillis, for one, seemed baffled and dispirited by Mr. Trump’s chaotic interference in the confirmation process, holding out hope that the president’s back-and-forth approach would end before the Senate went on summer recess early this week.

“It’s unfortunate that Todd Blanche, who I consider qualified for the job, will not be confirmed because of this reversal,” Mr. Tillis wrote on social media. “Hopefully, we can resolve this by Tuesday.”

There appeared to be a breakthrough on the matter late Sunday night when Mr. Blanche posted two documents: a signed memo in which he said that the Justice Department was formally rescinding an order that created the fund and an unsigned statement in which the agency clarified the terms of the tax protections.

Mr. Blanche’s message said that the documents had emerged from “good faith discussions” with senators, but as of Monday morning, Mr. Trump had not yet weighed in on the plan.