The White House has directed one of its former legal advisers not to testify to a congressional committee about special counsel Robert Mueller's investigation.
The Democratic-led House Judiciary Committee has issued a legal order for Donald McGahn to appear on Tuesday.
A member of the committee said an impeachment inquiry against President Donald Trump would be warranted if Mr McGahn does not testify.
Mr McGahn told Mr Mueller that Mr Trump repeatedly tried to thwart his inquiry.
The Department of Justice and White House both released statements on Monday arguing that Mr McGahn, the president's former counsel, was under no obligation to give evidence.
Mr Mueller's two-year investigation did not determine that Mr Trump conspired with alleged Russian attempts to sway the 2016 US presidential 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."
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."
But Judiciary Committee chairman Jerrold Nadler said Mr Trump was trying to block damaging testimony about him obstructing justice.
"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.
The White House direction to Mr McGahn, Mr Nadler added, was "just the latest act of obstruction from the White House that includes its blanket refusal to cooperate with this committee."
Meanwhile, Representative David Cicilline, a member of the Judiciary Committee, said on Twitter: "If Don McGahn does not testify tomorrow [Tuesday], it will be time to begin an impeachment inquiry of" President Trump.
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.
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