As tensions rose between the Republican president and the Democrats who control the House of Representatives, lawmakers also negotiated for future testimony by Special Counsel Robert Mueller on his Russia investigation.

The showdown between Trump and the Democrats intensified after McGahn, heeding Trump's instructions, ignored a subpoena from the Democratic-led House Judiciary Committee and did not testify before the panel.

Undeterred in a growing conflict with Trump over congressional powers to oversee his administration, committee Chairman Jerrold Nadler announced he had issued fresh subpoenas to Hicks, the former White House communications director, and to Annie Donaldson, McGahn's former chief of staff.

In the early days of Trump's presidency, few aides had more frequent access than Hicks, a former model and public relations consultant hired into the White House from daughter Ivanka's staff. She rose to communications director, but resigned from the White House in March 2018.

The subpoenas seek testimony and documents in connection with the committee's probe of whether the president obstructed Mueller's inquiry into Russian interference in the 2016 US election.

Despite McGahn's absence, the committee held a hearing lasting about 30 minutes that featured an empty chair at the witness table.

Nadler said at the hearing, "Let me be clear: this committee will hear Mr. McGahn's testimony, even if we have to go to court to secure it."

In Mueller's investigative report, McGahn was a key witness regarding possible obstruction of justice by Trump. 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.

Attorney General William Barr, the top US law enforcement official and a Trump appointee, on May 2 snubbed the same committee.

Republicans derided Tuesday's session as a political stunt.

Trump, seeking re-election in 2020, is refusing to cooperate with many congressional probes into his administration, family and business interests.

US District Judge Amit Mehta in Washington on Monday blocked a lawsuit by Trump that attempted to quash a subpoena sent by Cummings to Trump's long-time accounting firm Mazars LLP seeking the president's financial records. Trump has appealed the case.

Democrats have debated for months whether to initiate the impeachment process, with some lawmakers clamouring for it.

But senior leaders including House Speaker Nancy Pelosi have counselled caution for fear of a voter backlash that could benefit Trump.

Australian Associated Press