The White House has told a former adviser not to testify about Special Counsel Robert Mueller's investigation, sparking outrage from Democrats.
Lawyer Donald McGahn previously told the investigation into Russian interference in the 2016 election that he had felt pressured by President Donald Trump to fire Mr Mueller.
Mr McGahn has been ordered to appear on Tuesday.
But the White House has now directed him not to speak to the committee.
A letter to the Democratic-led House Judiciary Committee said lawyer Donald McGahn was "absolutely immune from compelled Congressional testimony".
There are calls for an impeachment inquiry against President Trump if he does not testify.
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.
Later 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.
What is the White House saying?
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."
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, Judiciary Committee chairman Jerrold 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.
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.
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.