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.
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.