Whitaker's testimony Friday was in limbo after the House Judiciary Committee approved a tentative subpoena to ensure he appeared.
Rep. Sheila Jackson Lee (D-Texas) snapped at Acting Attorney General Matthew Whitaker during his testimony before Congress Friday, telling him that making light of the proceedings was "not acceptable".
Late Thursday, there was considerable doubt about Whitaker's appearance.
Whitaker's hearing is probably going to be a contentious affair.
Whitaker's prepared testimony covered a range of Justice Department accomplishments, along with his intent not to discuss issues covered by executive privilege.
"I have not interfered in any way with the special counsel investigation", Whitaker told the committee. "That would have been considered a development that I would have been briefed on and I was briefed on that". The Senate judiciary committee voted along party lines Thursday to advance Barr's nomination.
It also followed a Thursday threat from Whitaker to not appear amid reports that Democrats were considering issuing a subpoena to compel him to answer particular questions.
Whitaker, in his opening remarks, said he hasn't given the White House any assurances about Mueller's probe, and the White House has not asked him to do so.
The rules of most hearings allow for each member to spend five minutes questioning a witness.
Nadler wrote in a letter to the Acting Attorney General, "To the extent that you believe you are unable to fully respond to any specific question, we are prepared to handle your concerns on a case-by-case basis, both during and after tomorrow's hearing".
Nadler and Democrats want to question Whitaker about how he got his job and the way he's overseen the investigation into Russia's attack on the 2016 presidential election being run by special counsel Robert Mueller.
On November 7, President Donald Trump announced the firing of Attorney General Jeff Sessions. But he did not hold back in his opening statement, repudiated Whitaker's decision not to recuse himself from the Mueller probe, despite the recommendations of Justice Department officials.
After Mr Whitaker, the former chief of staff to Mr Sessions, was temporarily appointed to the position, it emerged he had written several op-eds that were sharply critical of Mr Mueller's probe.
The blogger said the Democrats on the committee "are trying to redirect nearly every topic back to the Russian Federation investigation, and Whitaker has repeatedly said that he wouldn't answer questions about his private conversations with the president". Nadler said that is "ridiculous" and administration officials must provide the committee with answers or a better excuse to withhold them. "In the current environment, the American people have to know that there are places in government where the rule of law-not politics-holds sway, and where they will be treated fairly based exclusively on the facts and an even-handed application of the law", he said during his testimony last month.