WASHINGTON – Attorney General William Barr refused on Wednesday to testify before the House Judiciary Committee about special counsel Robert Mueller's investigation because of a dispute about how he would be questioned.

His refusal is the latest and most confrontational step by the Trump administration to challenge Democrats who control the House of Representatives and are conducting wide-ranging investigations into the president.

The dispute between Barr and lawmakers centers on how they planned to question him at a hearing that had been scheduled for Thursday. Judiciary Chairman Jerrold Nadler, D-N.Y., wanted the committee to include questioning by committee staffers, in addition to lawmakers.

But Republicans had argued that it would be unprecedented for staffers to question the attorney general as if he were being interrogated. And Barr had warned lawmakers that he would not attend the hearing if he would be questioned by staff members.

"He is trying to blackmail the committee," Nadler said. "We cannot allow the administration to dictate how we operate."

Barr spent four hours Wednesday defending his handling of Mueller's report during testimony before the Senate Judiciary Committee, where he clashed repeatedly with Democrats who complained that he misrepresented the report to benefit President Donald Trump.

Justice Department spokeswoman Kerri Kupec said Barr had volunteered to appear before the House committee, but that having staffers question him would be inappropriate.

"Unfortunately, even after the attorney general volunteered to testify, Chairman Nadler placed conditions on the House Judiciary Committee hearing that are unprecedented and unnecessary," Kupec said. "Chairman Nadler’s insistence on having staff question the attorney general, a Senate-confirmed Cabinet member, is inappropriate."

The Justice Department also did not comply with a subpoena for the unredacted version of the 448-page Mueller report. The committee had demanded that the department produce the full report by Wednesday.

"The next step is seeking a contempt citation against the attorney general," Nadler said. He said the committee could vote within days on whether to hold Barr in contempt for not turning over the full report.

"I understand why he wants to avoid that kind of scrutiny," Nadler said. He said Barr's refusal was in keeping with the administration's "complete stonewalling of Congress."

In a letter Wednesday to Nadler, the Justice Department's Assistant Attorney General Stephen Boyd said the committee has not "articulated any legislative purpose for its request" for documents relating to Mueller's investigation.

"Regrettably, before even reviewing the less-redacted version or awaiting the Attorney General's testimony, you served a subpoena demanding the unredacted report, every document cited therein, and 'all documents obtained and investigative materials created' by the Special Counsel's office over nearly two years," Boyd wrote. "The Committee has no legitimate role in demanding law enforcement materials with the aim of simply duplicating a criminal inquiry — which is, of course, a function that the Constitution entrusts exclusively to the Executive Branch."

Boyd railed against what he said was the aim of Democratic requests for evidence and other documents related to the inquiry. He said Democrats seemed to want to "second-guess prosecution and declination decisions made by the Department," and added, "This is not a legitimate use of congressional investigative authority."

Rep. Doug Collins of Georgia, the top Republican on the committee, said "Nadler chose to torpedo our hearing" rather than have the attorney general give "clear, informative testimony" as he had in the Senate on Wednesday. Collins said Democrats were trying to prolong the investigation that Mueller completed.

“Ultimately, though, they’re ignoring the will of the majority of Americans who want Congress to move on and secure our border and continue to strengthen our economy," Collins said.

The House session was expected to be less friendly, with a panel led by Democrats and nearly twice as many members as the Republican-led Senate panel.

The Senate hearing was dominated by questions about Barr's initial four-page summation of the Mueller report's conclusions and then the release of the redacted, 448-page report.

The Justice Department revealed Wednesday that Mueller had privately objected to Barr's initial summary of the investigation, which he said "threatened to undermine" the purpose of the probe.

Nadler said Barr had been misleading since at least March 24 and that staff lawyers could ask follow-up questions if Barr were to gives an unresponsive answer.

"He is terrified of having to face a skilled attorney," Nadler said. "I can understand why he is afraid of a more-effective examination."

Because Mueller's office declined to draw a conclusion about whether Trump had committed obstruction, the attorney general told the panel that he acted to resolve the question that had threatened to derail Trump's presidency. Nadler said he wanted Mueller to testify about his findings, and hoped to have the special counsel appear before the committee on May 15.

Contributing: Kevin Johnson and Christal Hayes, USA TODAY.

More about Attorney General William Barr and the Mueller report:

'America deserves better. You should resign': Key moments from William Barr's testimony

Collusion, obstruction of justice, redactions: How the Mueller report uses these legal terms

Mueller report: Trump's anger over Russia probe may have saved him from obstruction charge