By MARY CLARE JALONICK and LISA MASCARO Associated Press

WASHINGTON (AP) — House Judiciary Chairman Jerrold Nadler gaveled open a Trump-Russia hearing Tuesday with an empty witness chair and a stern warning that former White House Counsel Don McGahn will be held in contempt for failing to appear in defiance of the committee's subpoena..

"Our subpoenas are not optional," Nadler said. The panel will hear from McGahn "one way or another," he said. "This committee will have no choice but to enforce the subpoena against him."

Democrats are facing yet another attempt by President Donald Trump to stonewall their investigations . This time they're blocked from hearing from McGahn — a chief eyewitness to the president's handling of the federal Russia investigation — on orders from the White House.

Rep. Doug Collins, the ranking Republican on the Judiciary Committee, spoke scornfully of Nadler's position, calling the session a "circus" and saying the chairman preferred a public "fight over fact-finding."

Democrats are "trying desperately to make something out of nothing," Collins said, in the aftermath of special counsel Robert Mueller's findings in the Russia probe.

The committee voted to adjourn the hearing immediately after Collins' remarks.

A lawyer for McGahn had said he would follow the president's directive and skip Tuesday's hearing, leaving the Democrats without yet another witness — and a growing debate within the party about how to respond.

House Speaker Nancy Pelosi, backed by Nadler, is taking a step-by-step approach to the confrontations with Trump. Nadler said the committee would vote to hold McGahn in contempt, and take the issue to court.

"We will not allow the president to stop this investigation," the chairman said. A contempt vote is not expected until June, as lawmakers are scheduled to leave town for a weeklong recess.

Democrats are encouraged by an early success on that route as a federal judge ruled against Trump on Monday in a financial records dispute with Congress. Trump's team filed notice Tuesday that they would appeal.

But Pelosi's strategy hasn't been swift enough for some members of the Judiciary panel who feel Democrats should be more aggressive and launch a formal impeachment inquiry as they try to get information from the administration. Impeachment hearings would give Democrats more standing in court and could stop short of a vote to remove the president.

The issue was raised in a meeting among top Democrats Monday evening, where some members confronted Pelosi about it, according to three people familiar with the private conversation who requested anonymity to discuss it.

Maryland Rep. Jamie Raskin made the case that launching an impeachment inquiry would consolidate the Trump investigations and allow Democrats to keep more focus on their other legislative work, according to the people.

Pelosi pushed back, noting that several committees are doing investigations already and they have already been successful in one court case. But some members, several of whom have spoken publicly about a need to be more aggressive with Trump, are increasingly impatient. Other Democrats in the meeting siding with Raskin included Rhode Island Rep. David Cicilline, California Rep. Ted Lieu and freshman Colorado Rep. Joe Neguse.

Just before the start of Monday's meeting, Cicilline tweeted: "If Don McGahn does not testify tomorrow, it will be time to begin an impeachment inquiry of @realDonaldTrump."

In the hours after the discussion, Pelosi and Nadler met privately. Shortly afterward, Nadler said "it's possible" when asked about impeachment hearings.

"The president's continuing lawless conduct is making it harder and harder to rule out impeachment or any other enforcement action," Nadler said.

McGahn's refusal to testify is the latest of several moves to block Democratic investigations by Trump, who has said his administration will fight "all of the subpoenas." The Judiciary Committee voted to hold Attorney General William Barr in contempt earlier this month after he declined to provide an unredacted version of special counsel Mueller's report. And the House intelligence committee is expected to vote on a separate "enforcement action" against the Justice Department this week after Barr declined a similar request from that panel.

McGahn's lawyer, William Burck, said in a letter to Nadler that McGahn is "conscious of the duties he, as an attorney, owes to his former client" and would decline to appear Tuesday.

Still, Burck encouraged the committee to negotiate a compromise with the White House, saying that his client "again finds himself facing contradictory instructions from two co-equal branches of government."

McGahn was a key figure in Mueller's investigation, describing ways in which the president sought to curtail that federal probe. Democrats have hoped to question him as a way to focus attention on Mueller's findings and further investigate whether Trump did obstruct justice.

If McGahn were to defy Trump and testify before Congress, it could endanger his own career in Republican politics and put his law firm, Jones Day, in the president's crosshairs. Trump has mused about instructing Republicans to cease dealing with the firm, which is deeply intertwined in Washington with the GOP, according to one White House official and a Republican close to the White House not authorized to speak publicly about private conversations.

Administration officials mulled various legal options before settling on providing McGahn with a legal opinion from the Department of Justice to justify defying the subpoena.

"The immunity of the President's immediate advisers from compelled congressional testimony on matters related to their official responsibilities has long been recognized and arises from the fundamental workings of the separation of powers," the department's opinion reads.

A federal judge rejected a similar argument in 2008 in a dispute over a subpoena for Harriet Miers, who was White House counsel to George W. Bush. U.S. District Judge John Bates said it was an unprecedented notion that a White House official would be absolutely immune from being compelled to testify before Congress. Miers had to show up for her testimony, but still had the right to assert executive privilege in response to any specific questions posed by legislators, said the judge.

But in 2014, under the Obama administration, the Justice Department issued an opinion arguing that if Congress could force the president's closest advisers to testify about matters that happened during their tenure, it would "threaten executive branch confidentiality, which is necessary (among other things) to ensure that the President can obtain the type of sound and candid advice that is essential to the effective discharge of his constitutional duties."

Associated Press writers Michael Balsamo, Jonathan Lemire, Eric Tucker and Mark Sherman contributed to this report.