The House Judiciary Committee on Tuesday issued subpoenas to former White House officials Hope Hicks and Annie Donaldson for documents and testimony, setting the stage for another clash with the White House over former officials appearing before Congress.
McGahn was a key figure in Mueller's investigation, describing ways in which the president sought to curtail that federal probe.
In an extraordinary move, President Trump has vowed to block all subpoenas of his current and former staff. Even after Republican Rep. Justin Amash called for the president's impeachment over the weekend, House Majority Leader Nancy Pelosi said that she had not changed her mind about the downsides of an impeachment inquiry. Nadler said the committee would vote to hold McGahn in contempt, and take the issue to court.
The administration also claims that the Congress can not legally be compelled to testify, but Mr Nadler said he would hold Mr McGahn in contempt and pursue other means of compelling testimony. In his opinion on Monday, Mehta invoked the Senate's Watergate investigation, arguing "history has shown that congressionally-exposed criminal conduct" by the President or other high-ranking officials could lead to legislation.
Some other Democratic leaders, while backing Pelosi, signaled that a march to impeachment may at some point become inevitable. "I believe we need to begin an impeachment inquiry", agreed Escobar.
But Pelosi's strategy hasn't been swift enough for some members of the Judiciary panel who feel Democrats should be more aggressive and launch a formal impeachment inquiry as they try to get information from the administration.
Democratic Reps. Jamie Raskin (Md.), Joe Neguse (Colo.), and David Cicilline (R.I.) - all of whom are members of the House Judiciary Committee - reportedly joined Lieu in pressuring the House Speaker to support an impeachment inquiry.
"Mr. McGahn has a legal obligation to be here for this scheduled appearance", Nadler said.
"Our subpoenas are not optional", Nadler said at the start of Tuesday's hearing as he faced an empty chair reserved for McGahn at the witness table.
White House press secretary Sarah Sanders accused Democrats of pushing for "a wasteful and unnecessary do-over" of the Mueller report.
However, the Justice Department advised the White House that McGahn is immune to Nadler's subpoena.
"As I said earlier today, the Judiciary Committee's investigation into obstruction of justice, public corruption and abuse of power by President Trump and his Administration will continue", Nadler said in a statement. "It is simply not fathomable that a Constitution that grants Congress the power to remove a president for reasons including criminal behavior would deny Congress the power to investigate him for unlawful conduct - past or present - even without formally opening an impeachment inquiry", he maintained, specifically adding, "President Trump can not block the subpoena to Mazars". Career prosecutors not involved in the case have said the report contained strong evidence that Trump committed a crime when he pressured McGahn to fire Mueller and later urged him to lie about it. Trump attorney Jay Sekulow said that they "will be filing a timely notice of appeal to the D.C. Circuit Court of Appeals".
The redacted, 448-page Mueller report, 22 months in the making, showed how Moscow interfered in the 2016 USA presidential election in Trump's favor and detailed Trump's attempts to impede Mueller's probe.
Attorney General William Barr and others now are investigating how the Russian Federation probe began. "Agreeing with the Democrats" contention that they need the records because they are examining whether ethics and disclosure laws need to be strengthened, Judge Mehta said in a a 41-page ruling the justification was sufficient to make the subpoena valid.
Mr McGahn left the White House in October to return to a Washington law firm, Jones Day, which represents the Trump campaign.