House Judiciary Chairman Jerrold Nadler, D-N.Y., on Tuesday threatened to go to court to get former White House counsel Don McGahn to testify before his committee.

"Let me be clear: This committee will hear Mr. McGahn's testimony, even if we have to go to court to secure it," Nadler said.

Nadler's remarks came during an empty-chair hearing where McGahn was under subpoena to testify about special counsel Robert Mueller's Russia report. On Monday, President Donald Trump had directed McGahn not to comply with the subpoena, according to letters from administration officials who argued that McGahn was "immune" from being compelled to provide testimony.

"Our subpoenas are not optional," Nadler said in the brief hearing, which adjourned after remarks from him and ranking Republican Doug Collins, R-Ga.

McGahn's decision to skip the hearing only escalates the ongoing clash between House Democrats, who claim they are exercising their legitimate oversight duties, and Republicans who say the slew of subpoenas issued to figures related to Trump associates are intended to undermine the president.

"Democrats are trying desperately to make something out of nothing, which is why the chairman haphazardly subpoenaed today's witness," Collins said in prepared remarks.

Less than an hour after the hearing adjourned, the committee's vice chair, Rep. Mary Gay Scanlon, D-Pa., called for an impeachment inquiry against Trump. McGahn's absence, she said in a statement, represented an "inflection point" in the bitter back-and-forth between congressional Democrats and the White House.

"The President's refusal to produce evidence or permit witness testimony defies not only the rule of law but the basic protections of our Constitution," Scanlon wrote. "No one is above the rule of law."

White House counsel Pat Cipollone sent a letter to Nadler on Monday asserting that McGahn was "absolutely immune from compelled congressional testimony" due to his former position as Trump's senior advisor.

Nadler rejected that argument, citing what he said was a similar assertion of executive privilege by President George W. Bush in 2007 that was "slapped down" by Judge John Bates.

"In other words, when this committee issues a subpoena — even to a senior presidential advisor — the witness must show up," Nadler said. "If he does not immediately correct his mistake, this committee will have no choice but to enforce the subpoena against him."

In a CNN interview Monday evening, Nadler said he planned to have his committee take legal action against McGahn if he defied the subpoena.

"The first thing we are going to do we're going to have to hold McGahn in contempt" by the committee, Nadler told CNN. He said the full House would then move to enforce the contempt citation before launching court proceedings.

The committee had voted to hold another Trump administration official in contempt less than two weeks earlier. Attorney General William Barr, who defied a subpoena to hand over an unredacted version of the Mueller report and its underlying evidence.

McGahn is the most-cited witness in Mueller's report on Russian election meddling, possible obstruction of justice by Trump and possible coordination between the Kremlin and Trump's campaign. Mueller didn't find sufficient evidence to show Trump-Russia coordination, and Barr and former Deputy Attorney General Rod Rosenstein determined there was insufficient evidence for an obstruction offense.

But some of McGahn's interactions with Trump, detailed in the 448-page Mueller report, have been highlighted by Democrats claiming that the investigation shows significant evidence of wrongdoing by the president.

Nadler on Tuesday noted that Trump allegedly directed McGahn in 2017 to stop then-Attorney General Jeff Sessions from recusing himself from any probes of Russian election interference. Nadler also mentioned Trump allegedly told McGahn to order Rosenstein to fire Mueller.

McGahn refused to take either action.

"I believe that each of these incidents, documented in detail in the Mueller report, constitutes a crime," Nadler said of those and a handful of other incidents. "But for the Department of Justice's policy of refusing to indict a sitting president, I believe he would have been charged with these crimes."