The US House Judiciary Committee convened a hearing on Tuesday with another empty chair at the witness table, as former White House counsel Don McGahn refused to testify.

In a further escalation of a struggle between President Donald Trump and Congress over its power to investigate him, the White House on Monday told McGahn, who left his post in October, to disregard a subpoena from the Democratic-led House Judiciary Committee subpoena to appear at the hearing.

The panel is investigating Trump and Special Counsel Robert Mueller‘s inquiry into Russian election meddling. Attorney General William Barr on May 2 also snubbed the committee, which later voted to hold him in contempt of Congress for not handing over an unredacted copy of Mueller’s final report.

“Our subpoenas are not optional,” Jerrod Nadler, the chairman of the House Judiciary Committee said during the opening remarks of Tuesday’s hearing. “Mr McGahn has a legal obligation to be here for his scheduled appearance. If he does not immediately correct his mistake, this committee will have no choice but to enforce the subpoena against him.”

Trump is stonewalling numerous congressional inquiries into himself, his turbulent presidency, his family and his sprawling business interests, which he did not divest or put into a blind trust when he took office in January 2018.

Chairman of the House Judiciary Committee, US Representative Jerry Nadler, speaks during a hearing where former White House lawyer Don McGhan is expected to testify on the Mueller report [Mandel Ngan/AFP]

Trump and most fellow Republicans in Congress dismissed the inquiries as political harassment in advance of the 2020 elections.

Late on Monday, the Department of Justice issued a legal opinion saying McGahn did not need to appear at the hearing, while McGahn’s lawyer, William Burck, wrote that his client would not testify before the committee unless it reached an agreement with the White House.

In a letter sent to McGahn, Nadler told the former White House counsel that he would “risk serious consequences” if he failed to show up to testify.

“Should you fail to do so, the committee is prepared to use all enforcement mechanisms at its disposal,” Nadler wrote.

Trump appeals in financial records case

On another front, in a legal setback for Trump, a US judge on Monday ruled against him in a case involving another House panel. The House Oversight Committee has subpoenaed Trump’s financial records from his long-time accounting firm Mazars LLP.

In an unusual move, lawyers for Trump and the Trump Organization, his company, last month sued to try to block the subpoena. US District Judge Amit Mehta in Washington, DC, ruled against Trump and denied his request for a stay pending appeal.

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Early on Tuesday, Trump appealed the judge’s ruling, challenging “all aspects” of Mehta’s decision.

As the confrontation between Trump and Congress has intensified, Democrats have raised growing concerns about the president’s conduct, especially since the mid-April release of the Mueller report.

“We simply cannot sit by and allow this president to destroy the rule of law … If Mr McGahn doesn’t testify tomorrow, I think it is probably appropriate for us to move forward with an impeachment inquiry,” Democratic Representative David Cicilline, a Judiciary Committee member, told MSNBC on Monday.

The redacted, 448-page report from Special Counsel Robert Mueller, 22 months in the making, showed how Moscow interfered in the 2016 US presidential election in Trump’s favour and detailed Trump’s attempts to impede Mueller’s probe.

The report found there was insufficient evidence to allege a criminal conspiracy between Moscow and the Trump campaign. It made no recommendation on whether Trump obstructed justice, leaving that question up to Congress.