WASHINGTON — For years, President Trump has used Twitter as his go-to public relations weapon, mounting a barrage of attacks on celebrities and then political rivals even after advisers warned he could be creating legal problems for himself.
Those concerns now turn out to be well founded. The special counsel, Robert S. Mueller III, is scrutinizing tweets and negative statements from the president about Attorney General Jeff Sessions and the former F.B.I. director James B. Comey, according to three people briefed on the matter.
.. Several of the remarks came as Mr. Trump was also privately pressuring the men — both key witnesses in the inquiry — about the investigation, and Mr. Mueller is examining whether the actions add up to attempts to obstruct the investigation by both intimidating witnesses and pressuring senior law enforcement officials to tamp down the inquiry.
.. Trump’s lawyers said. They argued that most of the presidential acts under scrutiny, including the firing of Mr. Comey, fall under Mr. Trump’s authority as the head of the executive branch and insisted that he should not even have to answer Mr. Mueller’s questions about obstruction.
But privately, some of the lawyers have expressed concern that Mr. Mueller will stitch together several episodes, encounters and pieces of evidence, like the tweets, to build a case that the president embarked on a broad effort to interfere with the investigation. Prosecutors who lack one slam-dunk piece of evidence in obstruction cases often search for a larger pattern of behavior, legal experts said.
.. the nature of the questions they want to ask the president, and the fact that they are scrutinizing his actions under a section of the United States Code titled “Tampering With a Witness, Victim, or an Informant,” raised concerns for his lawyers about Mr. Trump’s exposure in the investigation.
.. “If you’re going to obstruct justice, you do it quietly and secretly, not in public,” Mr. Giuliani said.
.. federal investigators are seeking to determine whether Mr. Trump was trying to use his power to punish anyone who did not go along with his attempts to curtail the investigation.
.. Investigators want to ask Mr. Trump about the tweets he wrote about Mr. Sessions and Mr. Comey and why he has continued to publicly criticize Mr. Comey and the former deputy F.B.I. director Andrew G. McCabe, another witness against the president.
.. They also want to know about a January episode in the Oval Office in which Mr. Trump asked the White House counsel, Donald F. McGahn II, about reports that Mr. McGahn told investigators about the president’s efforts to fire Mr. Mueller himself last year.
.. Mr. Trump has navigated the investigation with a mix of public and private cajoling of witnesses.
.. Around the time he said publicly last summer that he would have chosen another attorney general had he known Mr. Sessions was going to recuse himself from the Russia investigation, Mr. Trump tried behind closed doors to persuade Mr. Sessions to reverse that decision. The special counsel’s investigators have also learned that Mr. Trump wanted Mr. Sessions to resign at varying points in May and July 2017 so he could replace him with a loyalist to oversee the Russia investigation.
.. Mr. Trump issued an indirect threat the next day about Mr. Comey’s job. “It’s not too late” to ask him to step down as F.B.I. director, he said in an interview with Maria Bartiromo on Fox Business Network. The special counsel wants to ask the president what he meant by that remark.
.. Mr. Sessions, his aide told a Capitol Hill staff member, wanted one negative article a day in the news media about Mr. Comey, a person familiar with the meeting has said.
.. By the fall, Mr. Comey had become a chief witness against the president in the special counsel investigation, and Mr. Trump’s ire toward him was well established. His personal attacks evolved into attacks on Mr. Comey’s work, publicly calling on the Justice Department to examine his handling of the Clinton inquiry — and drawing the special counsel’s interest.
.. Mr. Trump’s lawyers have pushed back against the special counsel about the tweets, saying the president is a politician under 24-hour attack and is within his rights to defend himself using social media or any other means.
.. The president continues to wield his Twitter account to pummel witnesses and the investigation itself, ignoring any legal concerns or accusations of witness intimidation.