They also asserted that "he could, if he wished, terminate the inquiry, or even exercise his power to pardon if he so desired."
Legal analysts said that as the head of the executive branch, Trump could issue pardons, fire senior officials or order them to shut down investigations. But if his motives were corrupt, such actions could constitute obstruction.
The principle laid out in the letter is "a ludicrous legal theory," said Neal Katyal, a former acting solicitor general who now works in private practice at Hogan Lovells. "The idea that a president can't obstruct justice died with King George III, with a brief attempt at revival by Richard Nixon."
The letter, signed by lawyers John Dowd and Jay Sekulow, was part of a bid to keep Trump from having to sit for an interview with Special Counsel Robert Mueller, especially on questions about whether he obstructed justice.
The lawyers asserted that Mueller's team had "an ample record upon which to base your findings of no obstruction," and thus there was no need for the President to talk with investigators.
"This memo is a polite way of taking 20 pages to say, 'He's not coming in without a subpoena, and even then, you're in for a protracted fight,' " Frenkel said.
Mueller is exploring whether the President meant to thwart law enforcement by, among other things, asking then-FBI Director James Comey not to pursue a probe of former National Security Adviser Michael Flynn, firing Comey, and pressuring the Attorney-General, Jeff Sessions, to either resign or take command of Mueller's probe. Sessions recused himself because of his role within the Trump campaign.
The letter aimed to rebut each piece of the case. The lawyers asserted that Comey's firing was justified because it was recommended by the Attorney-General and Deputy Attorney-General. They wrote that Trump disputes having told Comey to let go of the Flynn investigation, and that even if he had, the President could not have intended to obstruct justice because the White House had indications the Flynn investigation was not ongoing at the time.
The lawyers also made the technical argument that an FBI investigation, unlike a grand jury investigation, did not count under the law as a "proceeding" that could be obstructed. But that seemed to overlook a 2002 law that makes it illegal to obstruct even proceedings that have not yet begun.
THE SELF-PARDON
Many of the legal issues the President's team raised have little precedent, legal analysts said. No president, for example, has ever tried to pardon himself.
Rudy Giuliani, a lawyer for the President, said during television appearances today that while Trump "probably" had the power to pardon himself, he had no intention of doing so. The move, Giuliani said, could spark impeachment proceedings.
Jonathan Turley, a law professor at George Washington University, said he believed the President has the authority to pardon himself, and using pardons as a piece of an obstruction case might be problematic "because the nature of the act is to bar prosecutions or set aside convictions."
"That has been something of a parlour game for constitutional scholars for years, and many of the fun hypotheticals that we used to enjoy over beers all seem to be coming to fruition under the Trump Administration," Turley said.
Jed Shugerman, a Fordham University Law School professor, said raising the idea of pardons could be read as a warning to Mueller that if he pushes to subpoena Trump, the President could escalate the fight by pardoning people under investigation.
"There is a real practical consequence for the republic if Trump is able to win the silence of other defendants with the use of these pardons," Shugerman said. "If he's making an implicit threat that he could escalate this with pardons, there's nothing in the letter to indicate that he's not willing to pardon himself."
THE SUBPOENA
It is unclear, too, whether and to what extent Mueller might be able to compel Trump to testify, if the President will not agree to a voluntary interview. By virtue of his office, the President does enjoy special protections. The Justice Department has twice opined that a sitting president cannot be indicted, and Giuliani has said Mueller's team conveyed to the president's lawyers they will honour that guidance.
Those opinions, though, are not the same as a court decision, nor do they rule out a president being forced to testify. Independent counsel Kenneth Starr served President Bill Clinton with a subpoena to compel him to appear before a grand jury, but it was withdrawn after Clinton agreed to testify voluntarily.
Trump's lawyers pointed to case law suggesting Mueller would have to show that investigators were unable to obtain the same information elsewhere before demanding an interview with Trump. Turley said, though, that because an obstruction case hinges on intent, Trump might have a hard time convincing a judge that investigators could get that information without talking to him.
"In issues of intent, there's only one person on Earth that can conclusively speak to that issue, and that is the President himself," Turley said.
THE SUPREME COURT
If Trump were served with a subpoena, legal analysts said they expect he would challenge it, and the dispute would almost certainly escalate to the Supreme Court. They said that while the courts might ultimately limit what the President could be asked, it was unlikely they would agree with the President's legal interpretation that he cannot obstruct justice.
"For me, it is difficult to believe that any Supreme Court justice would endorse the right of a president to be lawless, in the sense of being entirely above the application of law," Frenkel said.
