As he pardoned D'Souza last week for a campaign finance law violation, Trump also said he was weighing commuting the prison sentence of former IL governor Rod Blagojevich, D, as well as granting a pardon to Martha Stewart, the television personality and lifestyle mogul, arguing that they and D'Souza had been unfairly treated by the justice system.
In the letter, which The New York Times published Saturday, Sekulow and Dowd argued the President could not possibly have committed obstruction in the Russian Federation probe because he has constitutional powers to "terminate the inquiry, or even exercise his power to pardon if he so desired".
He said there are obvious political reasons for a president to avoid a self-pardon.
Both Russia and Trump deny collusion, and the president has denied obstructing the probe.
His comment suggested that Trump's lawyers were beginning to persuade the president of the dangers involved.
Trump claimed last summer in another Twitter diatribe that as he President he has "the complete power to pardon".
And the letter also argues that the news media has not fully reported what is considered a damning statement by Trump to NBC anchor Lester Holt on why he fired Comey. Tell that to Bill Clinton.More news: Golden State Warriors v Cleveland Cavaliers: NBA finals Game 2
Levin said the status of Mueller's team violates the Appointments Clause under the Constitution, and publicly urged defendants called on to testify by the special counsel to challenge his authority in court.
His lead attorney, Rudy Giuliani, says the commander in chief "probably does" have the legal ability to take such an action. The letter lists 16 subject areas Mueller's team meant to question Trump about, including his firing of FBI Director James Comey.
Hours earlier, on ABC's Sunday Beltway show, Giuliani was asked whether can Trump pardon himself.
The letter was delivered to Mueller's office in January and authored by Jay Sekulow, who remains a member of the president's legal team, and John Dowd, who resigned from Trump's team in March.
Other experts made the same point, while others said the memo went too far.
"He tells America instantly ... how he feels about this investigation. A president can under extreme circumstances be indicted, but we're facing extreme circumstances".
"In no case can he be subpoenaed or indicted".
Long-standing Justice Department rules have concluded that a sitting president can not be indicted for criminal wrongdoing.