The House Judiciary Committee asked a federal judge on Wednesday to compel testimony from former White House counsel Donald McGahn, whom lawmakers consider their “most important” witness in any potential impeachment proceeding against President Trump.

McGahn figured prominently in special counsel Robert S. Mueller III’s investigation of whether Trump obstructed justice during the Justice Department’s probe into Russian interference in the 2016 election. The committee subpoenaed him in April, but the White House blocked his testimony, claiming McGahn, who left the administration in October, had “absolute” immunity.

Lawyers for the committee’s Democrats call the claim “spurious” and say it has no grounding in case law.

The complaint, filed in the U.S. District Court for the District of Columbia, escalates the months-long feud between congressional Democrats and the president. It marks the first lawsuit Democrats have filed to force a witness to testify since they regained control of the House last fall and launched investigations into the president’s conduct and finances.

The House has not voted to launch impeachment proceedings, but Judiciary Committee Chairman Jerrold Nadler (D-N.Y.) has said the panel is pursuing an impeachment investigation.

“Given McGahn’s central role as a witness to the president’s wide-ranging potentially obstructive conduct, the Judiciary Committee cannot fulfill its constitutional investigative, oversight and legislative responsibilities — including its consideration of whether to recommend articles of impeachment — without hearing from him,” the lawsuit says.

McGahn’s lawyer, William A. Burck, said in a statement that McGahn will abide by the president’s instructions absent a contrary decision from the court. McGahn, now a partner at Jones Day, “has an ethical obligation to protect client confidences,” Burck said. “Don does not believe he witnessed any violation of law. And the president instructed Don to cooperate fully with the special counsel but directed him not to testify to Congress unless the White House and the committee reached an accommodation.”

The committee’s top Republican, Rep. Douglas Collins (Ga), said in a statement that the Democrats’ lawsuit would stymie efforts to obtain additional documents from the White House and the Justice Department, and “further proves they are only interested in the fight and public spectacle of an investigation, but not actually in obtaining any real information.”

The lawsuit states that McGahn witnessed “nearly all of the most egregious episodes of possible presidential obstruction,” and his statements are mentioned in the special counsel’s 448-page report more than 160 times.

For instance, on June 17, 2017, three days after The Washington Post reported the special counsel was investigating whether the president had obstructed justice and a month after Mueller was appointed, Trump called McGahn at home twice and directed him to fire Mueller over alleged conflicts of interest, the complaint says, citing the report.

McGahn declined, advising the president that this “would be ‘another fact used to claim obstruction of justice,’ ” the lawsuit says.

McGahn again resisted when the president sought to have him issue a public statement and a “letter” for White House records rebutting media reports, published in January 2018, indicating that Trump had ordered McGahn to have Mueller fired the previous summer, according to the suit.

And he rebuffed the president’s effort in March 2017 to get him to pressure then-Attorney General Jeff Sessions not to recuse himself from the Russia probe, the lawsuit says. When McGahn declined, Trump “expressed anger at McGahn about the recusal,” the complaint says.

McGahn also was a “key witness” to the events leading up to Trump’s decisions to fire his first national security adviser, Michael Flynn, and then-FBI director James B. Comey in apparent attempts to end the Justice Department probe into Russia’s election interference and possible coordination with Trump associates, the complaint stated.

“McGahn is uniquely positioned to explain those events, bring additional misconduct to light, and provide evidence regarding the president’s intent,” the complaint says, noting that Trump has disputed significant portions of these events and accused McGahn of fabricating facts. “Live testimony from McGahn is essential” to resolving any conflicting accounts, it says.

The “absolute immunity” doctrine has no grounding in the Constitution, any statutes or case law and never has been accepted by any court, Democrats argued in the lawsuit, which was filed by House General Counsel Douglas N. Letter . The only federal judge to consider the issue was in the same courthouse more than a decade ago, the lawsuit notes. The judge in that case rejected the George W. Bush administration’s claim of immunity for then-White House counsel Harriet Miers. As a result, she was forced to testify before the House Judiciary Committee over her role in the controversial firings of U.S. attorneys.

House Democrats are sharply divided on the question of impeachment, with just over half, including Nadler, favoring proceedings. House Speaker Nancy Pelosi (D-Calif.) has argued that Democrats ought to use the courts to make their case against the president, and she has repeatedly pointed out that voters are less interested in impeachment than they are in issues like health care and the economy.

According to a recent poll, only about 37 percent of Americans favor impeachment.

The lawsuit follows the committee’s motion filed in the same court last month to force the Justice Department to turn over grand jury information related to the Russia probe, which concluded in March after nearly two years.

Committee lawyers have been negotiating with McGahn and the White House for months, trying to secure his testimony. They reached an agreement with the White House in recent days for McGahn’s documents but hit an impasse on his appearance, leading to the lawsuit, they said.

They said McGahn can offer critical details about Trump’s intent and “emotional state” during the episodes described by Mueller’s report, which would help establish whether the president meant to obstruct justice. A favorable ruling would enable not only McGahn’s testimony, but also that of several others, such as former communications director Hope Hicks, whose appearance the White House blocked with a similar argument, the lawyers said.

Rosalind S. Helderman contributed to this report.