Special counsel Robert S. Mueller III’s decision that he could not conclude whether President Trump’s attacks on the Russia investigation amounted to obstruction of justice illustrates the difficulty in proving such a charge — and left Democrats with a narrow but fraught path to press a case against Trump.

In the end, Trump’s mercurial behavior and relentless attacks on the FBI and special counsel probably extended the length of the probe — but the fact that many of his eruptions were in public view also may have made it more difficult to show he had ill intent, a key element in proving obstruction, legal experts said.

In his still-confidential report, Mueller stopped short of drawing a conclusion about whether the president obstructed justice, writing that “while this report does not conclude that the President committed a crime, it also does not exonerate him,” according to a summary by Attorney General William P. Barr released Sunday.

Barr went further, saying that he and Deputy Attorney General Rod J. Rosenstein concluded the evidence gathered by the special counsel “is not sufficient to establish that the President committed an obstruction-of-justice offense.”

That has left the question of Trump’s actionswhich included the firing of FBI Director James B. Comey, the public attacks on his attorney general Jeff Sessions, the regular cries of “witch hunt” and the taunting of witnesses — as one that will now be wrestled with in the political arena.

The obstruction question, as a legal matter, came down to whether the special counsel had evidence that Trump acted with “corrupt intent” as he demeaned and demonized the investigation.

With the term whirling around Washington, a former federal prosecutor explains what to know about the criminal charge of obstruction of justice. (Jenny Starrs/The Washington Post)

“Proving intent is genuinely difficult, because it requires you to get inside someone’s mind — and divining intent is art, not science,” said Chuck Rosenberg, a former U.S. attorney in Virginia and former counsel to Mueller.

Rosenberg said he was not surprised by the special counsel’s difficulties in drawing a conclusion on obstruction, noting that it is a complicated calculus to prove someone intended to block investigators and hide a crime.

“It was something we routinely debated, as prosecutors and agents,” he said.

In his report, Mueller addressed obstruction in an unusual way: He laid out evidence on both sides of the question but left “unresolved what the Special Counsel views as ‘difficult issues’ of law and fact,” Barr wrote in his summary. And Mueller pointedly did not clear the president.

One major factor in the Justice Department’s analysis: Since Mueller concluded that the Trump campaign did not conspire with Russia as it interfered in the 2016 campaign, prosecutors could not argue he acted corruptly to hide an underlying crime, legal experts said.

Prosecutors can seek a charge of obstruction when someone seeks to thwart a known criminal investigation, even if there wasn’t an underlying crime, some legal experts said, but they said it can often be difficult to then prove that case to a jury.

Compounding the challenge for Mueller was Trump’s refusal to sit for an interview in which prosecutors could have probed his motivations. Instead, his legal team provided only written answers from the president to Mueller’s questions about the campaign — not Trump’s activities in the White House.

And some of Trump’s actions — such as his firing of Comey — represented the exercise of powers afforded to the president by the Constitution, as his lawyers often argued.

To find criminal obstruction, a prosecutor must have evidence that a person’s actions would have the “natural and probable effect” of disrupting an investigation, said Mary McCord, former acting head of the Justice Department’s National Security Division, who oversaw the early stages of the investigation.

Barr wrote in his Sunday memo to lawmakers that Trump’s actions — many undertaken not furtively, but in full public view — didn’t reach that standard.

David Kris, who ran the national security division of the Justice Department in the Obama administration, said it was “notable” that Barr and Rosenstein came to a conclusion about the president’s actions within 48 hours of receiving Mueller’s final report, which was submitted late Friday.

“Attorney General Barr, in the space of a weekend, is able to make the judgment that Mueller precisely avoided making and described as being ‘difficult,’ ” said Kris, who now runs the consulting firm Culper Partners.

“On the merits then, Barr and Rosenstein together have reached the question that Mueller specifically avoided reaching, and they reached it very rapidly,” he said.

Always on offense

The attorney general’s assessment came after months in which Trump repeatedly sought to discredit the Russia probe — making public attacks that bewildered his staff.

Trump advisers often joked that the president’s actions made them think he was guilty, even as he said he was not. “Why would you behave that way if you’d done nothing wrong?” said one former senior administration official, who, like others, spoke on the condition of anonymity to describe internal discussions.

But Trump’s legal team long argued that Trump had no corrupt intent and that he was not intending to block the FBI inquiry into Russian activity, which began before he took office and was taken over by Mueller in May 2017.

Ty Cobb, a White House lawyer who advised Trump in the investigation’s first months, told others that the president was merely thin-skinned and he lashed out because he could not stand the idea that he was under scrutiny, according to people familiar with his comments.

Former New Jersey governor Chris Christie (R) said he warned the president repeatedly that his actions could only prolong the probe.

“He is always on offense. He never ever stops. Sometimes it works to his benefit, sometimes it works to his detriment,” Christie said Sunday. “I never thought it had anything to do with the underlying substance. I thought it was just who he was on the personal level.”

Trump’s actions provided a rich area of inquiry for the special counsel and his team. Comey has said that Trump told him in an Oval Office meeting in February 2017 that he hoped Comey would let go of an investigation into whether Michael Flynn, his former national security adviser, had lied about his contacts with the Russian ambassador. (Trump later denied he made that request.)

When Trump fired Comey, he told a Russian diplomat in the Oval Office and then an NBC News reporter that he did so with the Russia investigation in mind.

He then spent months on Twitter badgering Mueller, potential witnesses and Sessions, the attorney general who had earned Trump’s ire by recusing himself from the probe — and who was eventually forced out in November. He told aides at one point that he wanted to fire Mueller.

But Trump’s motives were difficult even for those closest to him to discern, because aides could not be certain if the president was ordering action or merely venting, said two people who testified to Mueller’s team about the incidents.

“He was always pushing back, pushing, pushing,” said one former senior administration official.

Question before Congress

Mueller’s failure to conclude whether Trump obstructed justice — followed by Barr’s quick assessment that he didn’t — now is likely to provide fodder for an already inflamed political debate about his actions as president.

“People are going to want to know what the facts were that weren’t good enough” to rise to the level of obstruction, said McCord, the former Justice Department official.

While in ordinary cases, the underlying evidence often should not be disclosed if it did not result in a charge, in the case of a high-level public official who is running for reelection next year, “the public has a right to know the facts” so they can make their own determinations about the official’s fitness to serve, she said.

In Congress, Democrats promised they would pick up where Mueller left off, insisting they must see the special counsel’s full report — and not just the brief Barr summary of his findings — to analyze the question.

House Judiciary Committee Chairman Jerrold Nadler vowed to immediately haul Barr to Capitol Hill to testify “in the near future” to explain his decision not to prosecute Trump.

The New York Democrat, whose committee has jurisdiction over any possible impeachment proceedings, highlighted Mueller’s specific language declining to “exonerate” the president.

Those words, he said, suggested that Justice Department officials are “putting matters squarely in Congress’ court” to continue to investigate.

Likewise, House Speaker Nancy Pelosi (D-Calif.) and Senate Minority Leader Charles E. Schumer (D-N.Y.) said in a joint statement that Barr — who wrote a memo prior to his appointment expressing expansive views of presidential power and criticizing Mueller’s obstruction investigation — was “not a neutral observer” and demanded Mueller’s report, as well as underlying investigative material, be made public.

But for Democrats, there could be political risk in pressing the question, particularly given the clean bill of health Mueller offered Trump on the question of whether he or anyone in his campaign conspired with Russia.

“The American people will long remember how wrong and irresponsible the Democrats have been,” House GOP Conference Chair Liz Cheney (Wyo.) said in a statement Sunday. “As we go forward, it’s time for Democrats to put aside their partisan agenda of attacking this President and instead focus on addressing the real issues facing the American people.”

Rachael Bade, Shane Harris, Carol D. Leonnig and Ellen Nakashima contributed to this report.