Mueller cited McGahn as saying that Trump called him several times in June 2017 to tell him to direct the Justice Department to remove Mueller because of conflicts of interest.
He said his committee would meet on Tuesday morning and expected McGahn to show up and testify. The investigation did, however, examine "multiple acts by the president that were capable of exerting undue influence over law enforcement investigations".
President Trump is preparing to instruct his former White House counsel, Donald F. McGahn II, to defy a congressional subpoena and skip a hearing scheduled for Tuesday, denying Democrats testimony from one of the most important eyewitnesses to Mr. Trump's attempts to obstruct the Russian Federation investigation, a person briefed on the matter said on Monday.
"The immunity of the President's immediate advisors from compelled congressional testimony on matters related to their official responsibilities has always been recognized and arises from the fundamental workings of the separation of powers", the opinion stated.
Meanwhile, Cipollone wrote to Judiciary Committee Chairman Jerry Nadler, D-N.Y., to affirm that McGahn would not appear at any hearing on Tuesday as Democrats had hoped.
That is still possible, since the justice department's legal opinion does not prevent McGahn testifying if he so chooses, although it would be potentially at risk to his own career.
Google-Huawei conflict won't affect Russian users, vows official
Despite these accusations, Huawei continues to maintain that is not possible for the Chinese government to poison its equipment. The upcoming devices will not have any access to the Play Store, first-party Google apps like Gmail, or Play Services.
The McGahn confrontation carries echoes of another former White House lawyer who was subpoenaed by Congress - Harriet Miers, a former adviser to President George W. Bush. "The Mueller Report documents a shocking pattern of obstruction of justice", he said in a statement.
Trump's lawyers, in cases from Washington and NY challenging the Democrats' demands, argued that congressional investigations are legitimate only if there is legislation that might result from them.
Another Democrat on the committee, David Cicilline, warned that if Mr McGahn stayed away, politicians might need to push ahead with an impeachment inquiry against Mr Trump in order to compel his administration to produce witnesses and documents. The Trump administration has blocked multiple requests and subpoenas from House Democrats, including one seeking tax returns, saying they serve no legislative goal.
Trump has denied any wrongdoing and said he did not ask McGahn to have Mueller removed. Democrats have long doubted the notion that Trump had fully handed over control over his vast estate and business operations to his sons.
McGahn featured prominently in Mueller's final report, released last month.
US Attorney General William Barr was questioned this month about the matter by Senate Democrats.
Barr also isn't giving the Judiciary Committee an unredacted copy of Mueller's report and its underlying evidence, as Nadler demanded with a subpoena.