Image copyright Reuters Image caption The judiciary committee had issued a subpoena to hear Mr McGahn's testimony, but his seat remained empty

A former White House counsel has failed to appear before Congress to testify about Special Counsel Robert Mueller's investigation, despite a subpoena.

House Judiciary Chair Jerrold Nadler has promised to take legal action.

President Donald Trump had earlier directed Donald McGahn to not appear before the Democratic-led committee.

Mr McGahn told investigators in the inquiry into Russian election meddling that he had felt pressured by the president to fire Mr Mueller.

"Our subpoenas are not optional," Mr Nadler said during his opening remarks on Tuesday. "This committee will hear Mr McGahn's testimony even if we have to go to court."

Mr McGahn could be held in contempt for defying the subpoena from Congress.

"We will not allow the president to block congressional subpoenas, putting himself and his allies above the law," Mr Nadler added.

Image copyright AFP Image caption Mr Nadler, the Democratic chair of the House Judiciary Committee, has promised legal action to hear from Mr McGahn

His Republican counterpart, ranking member Doug Collins, called the entire hearing theatrical and baseless.

"There was no collusion. There was no obstruction charge. There's nothing here," Mr Collins said. "The chairman rushed to maximise headlines by issuing a subpoena."

Mr McGahn served as White House counsel for nearly two years before his resignation in October 2018.

Both the Department of Justice and White House released statements on Monday arguing that Mr McGahn was under no obligation to give evidence.

A letter to the House of Representatives Judiciary Committee said Mr McGahn was "absolutely immune from compelled Congressional testimony".

There have been calls for an impeachment inquiry against President Trump if he does not testify.

Later on Monday, Mr McGahn's lawyer said his client would "respect the president's instruction".

Mr Mueller's two-year investigation did not determine that Mr Trump conspired with alleged Russian attempts to sway the 2016 election, but listed 10 instances of possible obstruction of justice by the president.

Image copyright AFP Image caption Don McGahn left the White House in October

What did the White House say?

White House press secretary Sarah Sanders said Democrats did not like the conclusions of the Mueller report and wanted "a wasteful and unnecessary do-over".

Citing the justice department guidance, her statement added: "The former Counsel to the President cannot be forced to give such testimony, and Mr McGahn has been directed to act accordingly."

Image copyright EPA Image caption Sarah Sanders said the White House had been "completely transparent with the Special Counsel's investigation"

In its memo, the justice department said Mr McGahn did not have to testify.

Assistant Attorney General Steven Engel said: "Congress may not constitutionally compel the president's senior advisers to testify about their official duties."

What has the reaction been?

Speaking to CNN, Mr Nadler said the panel would hold Mr McGahn in contempt of Congress for not testifying.

Earlier, he said the instruction was "just the latest act of obstruction from the White House that includes its blanket refusal to cooperate with this committee".

"The president acted again and again - perhaps criminally - to protect himself from federal law enforcement. Don McGahn personally witnessed the most egregious of these acts," he said in a statement.

However, Mr Nadler and House Speaker Nancy Pelosi are coming under growing pressure from their party to launch an impeachment inquiry against the president.

US media reports suggest several member of the judiciary committee tried to convince Ms Pelosi to start such an investigation to make the Trump administration comply with subpoenas.

Image copyright Getty Images Image caption Democratic Party members are putting Ms Pelosi under increasing pressure to start an impeachment inquiry

Representative David Cicilline, a committee member, said on Twitter: "If Don McGahn does not testify tomorrow [Tuesday], it will be time to begin an impeachment inquiry of" President Trump.

But Ms Pelosi and Mr Nadler told colleagues their course of action was getting results. Someone in the meeting told NBC News it was a "long and very emotional" debate.

Also on Monday, a federal judge rejected Mr Trump's efforts to block a subpoena into his accounting firm, Mazars USA LLP.

The subpoena, issued by the House Committee on Oversight and Reform on 15 April, asked that the firm hand over financial records relating to Mr Trump dating back to 2011, years before he announced his candidacy for president.

What is this about?

The subpoena for Mr McGahn's testimony is part of a wider inquiry by Congressional Democrats into Mr Trump's alleged obstruction and abuse of power.

In March, the House Judiciary Committee issued document requests related to the investigation to 81 people and groups.

Media playback is unsupported on your device Media caption The Mueller report - in 60 seconds

Mr McGahn was interviewed for 30 hours by Mr Mueller's team of investigators, and was frequently cited in their 448-page report, released in April.

Mr Trump later maintained he had authorised this co-operation with Mr Mueller.

The Mueller report detailed how Mr McGahn felt the president had pressured him to fire Mr Mueller and, later, write a memo saying that Mr Trump had issued no such directive.

US Attorney General William Barr was questioned this month about the matter by Senate Democrats.

He said the president had only suggested that Mr Mueller be "replaced" because of a perceived conflict of interest - and then instructed Mr McGahn to correct inaccurate media reports.

Mr McGahn left the White House in October to return to a Washington law firm, Jones Day, which represents the Trump campaign.