WASHINGTON – The White House's former top lawyer, Don McGahn, defied a congressional subpoena and skipped a hearing Tuesday where lawmakers had planned to press him on President Donald Trump's efforts to thwart the investigation of Russian interference in the 2016 election.

McGahn's refusal to comply with a House Judiciary Committee subpoena came at the direction of the White House and after a legal opinion from the Justice Department on Monday said that he could not be forced to appear before the panel.

The committee's chairman, Rep. Jerrold Nadler, D-N.Y., bristled at McGahn's absence.

“This conduct is not remotely acceptable,” he said Tuesday, facing an empty witness chair. “Our subpoenas are not optional.”

McGahn's move marked the latest in a series of clashes between Trump and lawmakers seeking to investigate him. The committee already found Attorney General William Barr in contempt for refusing to provide Congress with a complete version of special counsel Robert Mueller's report about the Russia inquiry. And Nadler said Tuesday that he intended to pursue McGahn's testimony "even if we have to go to court to secure it."

McGahn provided prosecutors with hours of testimony about Trump's efforts to stymie Mueller's investigation, and Democrats who lead the committee wanted to press him to detail those episodes in public.

But the department said Monday that McGahn need not appear to answer those questions. In a 15-page letter, the head of the department's Office of Legal Counsel, Steven Engel, told White House Counsel Pat Cipollone that McGahn and other advisers to the president cannot be compelled to testify.

"We provide the same answer the Department of Justice has repeatedly provided for nearly five decades: Congress may not constitutionally compel the president's senior advisers to testify about their official duties," Engel wrote. "Those principles apply to the former White House Counsel. Accordingly, Mr. McGahn is not legally required to appear and testify about matters related to his official duties as counsel to the president.”

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Republicans on the committee questioned the need to force McGahn to testify, rather than negotiating with the administration.

“Everything else has become a race to get a headline,” said the panel's top Republican, Rep. Doug Collins of Georgia. “Don’t undercut congressional oversight because you can’t wait.”

After about 20 minutes of statements from Nadler and Collins, the committee voted 21-14 to adjourn with no testimony from McGahn and no announcement of how it would try to secure his appearance in the future. Nadler has said previously that McGahn could be held in contempt.

“This is disgraceful,” Rep. Steve Chabot, R-Ohio, said in voting against adjournment.

Trump said Monday that the administration was blocking McGahn's testimony to protect the presidency rather than for his personal benefit.

"They're doing that for the office of the presidency," Trump said. "It's a very important precedent. They're not doing that for me."

A key portion of the Mueller report described how Trump tried to remove Mueller, as described by McGahn. Since the report was released, Trump has denied that he tried to fire Mueller. Lawmakers are eager to question McGahn about the discrepancy.

Nadler has warned McGahn's lawyer that the committee would have no choice but to find McGahn in contempt if he fails to testify and provide a log of unreleased documents.

But Cipollone earlier said in a letter to McGahn's lawyer, William Burck, that McGahn was working on the president’s behalf during the investigation so that the documents “remain legally protected from disclosure under longstanding constitutional principles.”

Trump has asserted executive privilege over the redacted portions of Mueller’s report and millions of pages of underlying evidence about Russian interference in the 2016 election, as a precaution while Justice Department officials review which documents should remain secret permanently. The probe ended with Mueller finding no conspiracy between the Trump campaign and Russia, and deciding not to pursue charges of obstruction of justice.

Trump allowed McGahn to meet with investigators for 30 hours during the course of the nearly two-year inquiry. Trump now contends that any documents McGahn could provide the committee are White House property and should remain secret. Trump has also said he will fight all House subpoenas as presidential harassment rather than legitimate congressional oversight.

Engel wrote that the executive branch has asserted for 75 years that senior presidential advisers may decline to testify before Congress, and formal immunity has been formally asserted for nearly 50 years. A U.S. District Court ruled in 2008 that presidential advisers weren’t entitled to absolute immunity, but the White House and Congress settled the case before an appeals decision was reached.

The Mueller report, based in part on testimony from McGahn and other senior Trump aides, said Trump called McGahn at home June 17, 2017, and told him Mueller had conflicts and should be removed. McGahn didn’t carry out the direction, however, deciding that he would resign rather than trigger what he regarded as a potential Saturday Night Massacre, a reference to former President Richard Nixon firing prosecutors during the Watergate investigation. Trump later met with McGahn in the Oval Office and pressured him again, but McGahn refused.

McGahn later told Trump's chief of staff that the president had asked him to "do crazy s---," according to the report.

Nadler said McGahn began speaking about a year ago to Mueller's investigators, describing the instances in which Trump directed him to have Mueller fired and “how the president ordered him to lie about it.” In contrast, he said, Trump refused to be interviewed.

More on President Donald Trump's legal battles with Congress:

'Slow-motion constitutional car crash': Trump, Congress battle over investigations with no end in sight

President Donald Trump tees up the strangest test yet of executive privilege

Federal judge refuses to block House subpoena for Trump's financial records

This article originally appeared on USA TODAY: Don McGahn, former White House counsel, defies House subpoena and skips hearing on Russia probe