By Ashley Parker, Philip Rucker, Tom Hamburger, Robert Costa and Matt Zapotosky | The Washington Post

WASHINGTON — The grand jury witnesses arrive one by one at the windowless room in the federal courthouse on Constitution Avenue in downtown Washington. They are struck first by how commonplace the setting feels – more classroom than courtroom, two witnesses said.

One of special counsel Robert Mueller III’s prosecutors stands at a lectern. The jurors, diverse by age and ethnicity, are attentive and take notes. The questioning is polite yet aggressive, surprising witnesses with its precision and often accompanied by evidence – including text messages and emails – displayed on a large old-fashioned overhead projector.

The investigation of Russian interference in the 2016 election, which hits its one-year mark Thursday, has formed the cloudy backdrop of Donald Trump’s presidency — a rolling fog of controversy, much of it self-inflicted, that is a near-constant distraction for the commander in chief.

The Mueller operation, like the former Marine Corps platoon commander who leads it, is secretive and methodical. Ten blocks west in the White House, President Trump combats the probe with bluster, disarray and defiance as he scrambles for survival.

The president vents to associates about the FBI raids on his personal attorney Michael Cohen – as often as “20 times a day,” in the estimation of one confidant – and they frequently listen in silence, knowing little they say will soothe him. Trump gripes that he needs better “TV lawyers” to defend him on cable news and is impatient to halt the “witch hunt” that he believes undermines his legitimacy as president. And he plots his battle plans with former New York Mayor Rudy Giuliani, his new legal consigliere.

“We’re on the same wavelength,” Giuliani said. “We’ve gone from defense to offense.”

The probe is a steaming locomotive, already delivering indictments or guilty pleas involving 19 people and three companies, while soliciting interviews with most of the president’s closest aides and outside associates. Players have departed, including most of Trump’s original legal team, while others have joined — including, most recently, Cohen, adult-film actress Stormy Daniels and her attorney, Michael Avenatti.

“This has moved at a lightning speed,” said Christopher Ruddy, a Trump friend and chief executive of Newsmax. “They’re not messing around. They’re going very quickly. The number of indictments, pleas and other moves is just amazing. I think it will come to a head quicker than other investigations.”

This portrait of the president and the special counsel investigation nearing its first anniversary is based on interviews with 22 White House and Justice Department officials, witnesses, Trump confidants and attorneys connected to the probe, many of whom spoke on the condition of anonymity to offer candid assessments.

“Everyone seems resigned to just buckle up and get through whatever we’ve got to get through for it to reach its conclusion,” one White House official said.

Many Trump aides and associates say they are confident the president himself will ultimately be exonerated. But they privately express worries that the probe may yet ensnare more figures in Trump’s orbit, including family members. There is particularly worry about Trump’s eldest son, Donald Trump Jr., and Jared Kushner, his son-in-law and a senior adviser.

Mueller’s investigation into Russia’s interference in the election and connections to Trump’s campaign and associates already has resulted in a guilty plea from former national security adviser Michael Flynn, who is cooperating, and an indictment of former campaign chairman Paul Manafort, who is scheduled to go on trial in Virginia in July and in Washington in September on conspiracy, bank fraud and tax fraud charges.

The special counsel also is examining whether Trump obstructed justice in a variety of areas, from his request of then-FBI Director James Comey to drop the Flynn investigation to his firing of Comey to his role dictating a misleading statement on behalf of Donald Trump Jr. about his 2016 Trump Tower meeting with a Russian lawyer.

Mueller and his prosecutors are probing other areas as well, including the relationship between former Trump political adviser Roger Stone and WikiLeaks founder Julian Assange, whose organization released hacked Democratic Party emails, according to people familiar with the probe.

The sprawling investigations amount to a political anchor as Trump leads the Republican Party into the fall midterm elections. Though few candidates see it as a decisive issue, the probe still sows doubt among some voters about the credibility of Trump’s election and about his conduct in office.

Public opinion surveys have found wide support for the Mueller investigation. An April Washington Post-ABC News poll found 69 percent of Americans backing the probe and 25 percent opposing it, though other surveys this spring have shown a modest decline from earlier polls in support of continuing the investigation.

Among the political class, there is a guessing game about whether the special counsel completes its work this summer – sufficiently in advance of the November elections – or presses well past it. The longer Mueller’s work continues, legal analysts said, the more difficult it may be for the special counsel to maintain public confidence, especially with Trump, Vice President Mike Pence and other administration officials calling for the probe to wrap up.

“You don’t have much longer than 18 months to 24 months to get to the heart of the matter and resolve the things that need to be resolved,” said Robert Ray, who served as independent counsel toward the end of the Whitewater investigation during the Clinton presidency. “That’s about the length of time that public sentiment is with the investigation.”

The Mueller probe has also brought a national reckoning about the boundaries of presidential power. Trump is at war with the leadership of his own Justice Department and FBI, has threatened to defy a subpoena to testify, and even toyed with ordering the firing of Mueller.

“We want to get the investigation over, done with,” Trump said last month. “Put it behind us.”

Mueller – the 73-year-old former FBI director with a hangdog visage and rigid bearing – looms over the investigation but is an intermittent presence in the windowless room in the E. Barrett Prettyman Federal Courthouse.

Three witnesses who described their experience of being subpoenaed to appear before the grand jury said Mueller was not present for their initial interviews, which instead were conducted by one of his prosecutors standing at a podium – peppering them with questions and presenting the case to members of the jury, who scribbled notes.

The cramped room, complete with inelegant furniture, one witness said, “looked like a classroom from an underfunded junior college in the 1970s.”

The range of witnesses Mueller has called in has been breathtaking. He has interviewed everyone from White House counsel Donald McGahn – at least twice – to Avi Berkowitz, the 29-year-old personal assistant to Kushner.

One prominent witness who was called to appear in front of the grand jury recounted entering through a rear entrance, to avoid the press gathered at the front of the building. But another, former Trump campaign aide Sam Nunberg, said he was not given that option and, regardless, preferred to enter and exit in full view of reporters.

“If they had asked me to go through a back door, I would have said, ‘No, I prefer to see the paparazzi,'” he said, recalling the phalanx of cameras that swarmed him during his appearance in early March.

Yet aside from a few witnesses who have shared glimpses of their experiences with Mueller’s team, the exact contours of the investigation remain opaque – even for Trump’s lawyers, who have been in regular contact with Mueller’s investigators.

Only last week, for instance, did the public learn that Mueller had been probing payments made by Fortune 500 companies to Cohen since at least last fall.

Mueller and his team seldom issue public statements and speak mainly through indictments and court filings. In pressing for an interview with Trump, investigators would not provide a written list of questions, which could increase the chances of a leak and constrain prosecutors in their inquiries. Instead, investigators verbally provided the president’s lawyers with only the subject areas that prosecutors wished to discuss. A Trump attorney then formulated a list of 49 potential questions the legal team believed Trump might be asked – a list that soon leaked to the New York Times.

“The biggest challenge for the White House is that the special counsel is conducting an investigation properly, which is not commenting publicly, only making known its activities by virtue of bringing cases or executing legal process in a manner that is publicly observable,” said Jacob S. Frenkel, who worked in the independent counsel’s office in late 1990s.

Even Giuliani, who said he was brought in to end the probe and initially predicted it would wrap up within two weeks, now seems uncertain of where Mueller’s investigation will conclude.

Giuliani met with Mueller five days after his hiring, on April 24, to try to understand issues ranging from the scope of a possible Trump interview to whether Mueller believes that Comey, whose firing by Trump triggered the probe, is a credible witness.

“From our point of view, it’s a two-track possibility for what’s next,” Giuliani said, referring to the possibility that Trump may sit for an interview with Mueller or, if he refuses, that Mueller may subpoena him. “But we don’t know which track it’ll end up being.”

Few achievements make Trump more proud than the 306 electoral college votes he won on Nov. 8, 2016. The president relishes showing off a county-by-county map of the election results – the United States bathed in red – and giving visitors a tour of the trophy he inherited, the Oval Office.

But every time he hears about “this Russia thing,” as he memorably phrased it in an NBC interview last year, he feels the legitimacy of his victory is under attack. He characterizes the Russia probe as a “hoax” orchestrated by Hillary Clinton and the Democrats – a reminder of the majority of voters who didn’t choose him and those who are eager to evict him.

The only option, the president has said, is to hit back.

“Let me tell you, folks, we’re all fighting battles,” Trump bellowed at the National Rifle Association annual convention earlier this month. “But I love fighting these battles.”

It would be easy to interpret the president’s tweets – and even his behavior – as an admission of guilt. But Trump’s advisers and friends say he believes he has done nothing wrong. What some legal analysts call obstruction of justice, Trump’s associates call punching back.

“His view is, ‘If I’m defending myself, you mean that’s obstructing justice?’ ” Giuliani said. “He’s right. He’s being president, but he’s not going to just sit there.”

Ruddy, who often talks with Trump during the president’s getaways to his Mar-a-Lago estate, said he would counsel him to wall himself off and emotionally disconnect from the investigation.

“People will say he’s acting like he’s guilty,” Ruddy said. “No. This is Donald Trump’s personality. He just has to respond. He’s been so emotional … It takes a toll on him, and the way he deals with it is to lash out.”

Trump’s attacks on Mueller and his probe are also helping to undermine the investigation in the court of public opinion, and especially with the president’s base.

“I don’t see any downside at this point for the president and his team to make a full-throated public defense of their situation,” said Mark Corallo, a former adviser on Trump’s legal team. “There are very few outside the Beltway who are in the we-need-to-prosecute-and-impeach-this-guy camp.”

Giuliani’s hiring marked the latest stage of the Russia fight. Already, Trump’s legal team was in flux. Ty Cobb, the White House lawyer dealing with the probe, had repeatedly counseled Trump that if he cooperated fully it would be over soon – first by Thanksgiving, then by Christmas, then early in 2018. But Cobb is now exiting, to be replaced by Emmet Flood, one of Clinton’s former impeachment attorneys. Also gone is John Dowd, who had been Trump’s personal lawyer and grew frustrated with his difficult client.

Trump liked Giuliani’s more aggressive approach, including his earlier television defenses of him. And the president, feeling increasingly isolated in the West Wing, with few true confidants on the staff, saw in Giuliani a loyal contemporary.

But within the White House, Giuliani – who already has a strained relationship with Kelly – has created tensions with other senior staff members, in part over his frequent media appearances, which he does not coordinate with them.

In a freestyling interview earlier this month with Fox News Channel host Sean Hannity, Giuliani disclosed that Trump had reimbursed Cohen for a $130,000 payment to Daniels near the end of the 2016 campaign in exchange for her silence about an alleged sexual encounter with Trump a decade earlier. The revelation drove headlines for days, frustrating the president, who told reporters that Giuliani was “a great guy” but needed to “get his facts straight.”

But for now, Trump and Guiliani are inextricably bound.

The two men huddled for five hours May 6 at Trump National Golf Club in Virginia, Giuliani said, eating a Cobb salad (Giuliani) and a well-done burger (Trump) with half a bun in service to his health.

“I do that, too, sometimes,” Giuliani said about the half-bun. “It’s a good way to do it.”

That afternoon, the lawyer said he counseled his client to focus on his job as president and leave the legal matters to him.

But Trump could not be restrained. The next day at 7:27 a.m., he fired off a presidential missive on Twitter: “The Russia Witch Hunt is rapidly losing credibility …”

The Washington Post’s Rosalind S. Helderman, Josh Dawsey and Carol D. Leonnig contributed to this report.