The panel wants to question Mr. Kline about the accusations made by a whistle-blower, Tricia Newbold, who has told the committee that senior Trump administration officials had issued security clearances to at least 25 officials whose applications were denied by career staff members who cited “disqualifying issues” that posed possible national security risks. The White House instructed Mr. Kline not to cooperate.

“Your refusal to participate in a deposition last Tuesday pursuant to a duly authorized subpoena from this committee is a very serious matter that places you in significant legal jeopardy,” Mr. Cummings warned in a letter he sent Saturday to Mr. Kline, adding, “I understand that the White House ordered you not to appear, but that is not a valid legal reason to defy a congressional subpoena.”

Representative Jim Jordan of Ohio, the top Republican on the oversight panel, has since intervened and worked to arrange for Mr. Kline to testify voluntarily. In his letter, Mr. Cummings scheduled a voluntary hearing for Wednesday, but made clear that he would not allow Mr. Jordan or the White House to set the parameters for questions.

That was a sentiment Mr. Nadler echoed in his response to Mr. Barr. The attorney general is scheduled to appear on Capitol Hill twice this week; on Wednesday, he is to come before the Republican-led Senate Judiciary Committee. But he is almost certain to face a tougher time in the Democrat-led House.

Democrats are up in arms over what they regard as his failure to accurately portray the Mueller report.

Both in a four-page written statement summarizing the report and at a news conference before releasing it to the public, Mr. Barr emphasized that Mr. Mueller had found no evidence that the Trump campaign had conspired with Russia to interfere in the 2016 elections, while playing down the findings that suggested the president may have obstructed justice. During the news conference, Mr. Barr repeatedly said there was “no collusion” — the president’s favorite phrase in defending himself.

Democrats want to know why Mr. Barr handled the release the way he did, what conversations he had with Mr. Mueller before the release and what he told the White House about it. And they will undoubtedly grill Mr. Barr on his conclusion, as articulated in his written statement, that the evidence that Mr. Mueller amassed was “not sufficient to establish that the president committed an obstruction-of-justice offense.”