And when Mr. Nadler began questioning Mr. Whitaker, the acting attorney general refused to provide details about when and how many times he had been briefed about the Russia investigation, which is led by the special counsel, Robert S. Mueller III. Then, when Mr. Nadler asked Mr. Whitaker if he had ever been asked to approve any request for action to be taken by Mr. Mueller, Mr. Whitaker audaciously replied: “Mr. Chairman, I see that your five minutes is up.”

The room broke into laughter as Mr. Nadler looked up in apparent disbelief at the breach of decorum, but then grinned himself and let it slide, noting that he did not enforce the five-minute rule during Mr. Whitaker’s opening statement. Mr. Nadler then directed Mr. Whitaker to “answer the question, please.”

Mr. Whitaker’s retorts to lawmakers soon irked Democrats. “Mr. Attorney General, we are not joking here,” said Representative Sheila Jackson Lee, Democrat of Texas, during one exchange. “And your humor is not acceptable.”

But as a matter of substance, the takeaway was that Mr. Whitaker testified under oath that he had never used his position to provide inside information about the Russia inquiry to Mr. Trump or his proxies, nor had he taken any step to impede Mr. Mueller’s work — both fears that Democrats have repeatedly expressed since Mr. Trump installed him as acting head of the department.

“I want to be very specific about this, Mr. Chairman, because I think it will allay a lot of fears that have existed among this committee, among the legislative branch, largely, and maybe amongst some American people,” Mr. Whitaker said. “We have followed the special counsel’s regulations to a T. There’s been no event, no decision that has required me to take any action, and I’ve not interfered in any way with the special counsel’s investigation.”

Mr. Whitaker pointedly declined at multiple points, though, to defend Mr. Mueller and his investigation from accusations by Mr. Trump or others that he was conducting a “witch hunt.”