Whitaker testified following a lengthy fight with Nadler over the prospect that he would be subpoenaed for not answering questions.
The Texas Democrat was trying to ask Whitaker if he had a confirmation hearing and if it has been more than ten years since he has testified before Congress.
Differences over a barrier were the crux of a dispute between Democrats and Trump that led to the recent partial government shutdown - and the government could shut down again unless negotiators can make progress on a deal to fund the Department of Homeland Security.
The acting attorney general first challenged Nadler's line of questioning when the chairman asked whether he had been briefed on Mueller's investigation in December 2018, including just before announcing that he would not recuse himself from the probe.
Representative Doug Collins of Georgia, the panel's top Republican, said Democrats were staging a "dog-and-pony show" simply "to damage the president". But he did not answer directly about whether he had discussed that investigation, run by prosecutors in NY, with Trump, insisting only that the president had never instructed him to take particular actions.
"[Democrats] are going so far to the left, so radical, so rude that I think the American public is gonna wake up", she said.
Acting AG Whitaker tells House Judiciary...
"Congresswoman, I am an acting attorney general". When pressed on Friday if he considered the investigation a witch-hunt, he said it would be "inappropriate" to comment on an ongoing investigation.
Whitaker refused to come before the House Judiciary until its Chair, Rep. Jerrold Nadler, announced he would serve Whitaker a subpoena if he didn't willingly testify.
"I intend to be fully transparent about this process", Nadler said. Collins said. "The key difference today is simply that this witness is part of the Trump Administration - and now we're setting a unsafe precedent".
It was when Whitaker reminded Rep.
"When career officials at the department recommended that you take steps to mitigate your apparent conflicts of interest, . you ignored them", Nadler said.
White House officials kept an eye on Whitaker's performance and, while they appreciated his combative tone and aggressive defense of the administration, there was a sense from aides that his performance, at times, appeared halting and ill-prepared. When Nadler gaveled the hearing into a five-minute recess, Whitaker said into an open mic: "Five minutes for lunch?"
She went on to bring up his conversations with President Donald Trump and asked if they ever spoke about the SDNY case involving Michael Cohen.
House Democrats had threatened to subpoena Whitaker over his oversight of the ongoing Russian Federation probe led by special counsel Robert Mueller, his contacts with White House related to the probe and the firing of former Attorney General Jeff Sessions.
"I will not allow the witness to stall and waste members' time", Nadler said.