WASHINGTON — The House Judiciary Committee took another step toward possible impeachment proceedings, filing a lawsuit in federal court on Wednesday aimed at forcing former White House counsel Donald McGahn to testify about his interactions with President Donald Trump.

McGahn was a star witness in special counsel Robert Mueller's report who — under Trump's orders — has refused to testify before the panel. The Democratic lawsuit challenges the White House rationale that McGahn and other witnesses have "absolute immunity" from appearing and can defy subpoenas.

The legal action comes at a time when more than half of House Democrats have said they support beginning an impeachment inquiry. Pelosi has so far resisted that step, saying she wants to wait to see what happens in court. The McGahn lawsuit is a central part of Pelosi's strategy of "legislate, investigate, litigate," but could delay any final decisions on impeachment for several months.

In a letter to colleagues announcing that the lawsuit was imminent, Pelosi said "no one is above the law."

The lawsuit says the Judiciary panel is "now determining whether to recommend articles of impeachment" based on Mueller's report. It says McGahn is "the most important witness, other than the president, to the key events that are the focus of the Judiciary Committee's investigation."

The complaint adds: "Every day that the Judiciary Committee is without McGahn's testimony further delays its ability to pursue its inquiries on issues of national importance before the current Congress ends."

The lawsuit says the committee has reached a deal with the White House to review documents from McGahn, but it is still seeking his testimony in person.

McGahn's lawyer, William A. Burck, in a statement said "McGahn is a lawyer and has an ethical obligation to protect client confidences" and does not believe he witnessed any violation of law.

"When faced with competing demands from co-equal branches of government, Don will follow his former client's instruction, absent a contrary decision from the federal judiciary," Burck said.

In a contrast to Pelosi, House Judiciary Committee Chairman Jerrold Nadler insists that the committee is essentially already doing the work of impeachment, with or without a formal House vote to begin an inquiry. Nadler has also made it clear that he'd favor beginning official proceedings, saying last month that Trump "richly deserves" impeachment.

Nadler laid out an aggressive timeline this week, saying he hopes to be able to have resolution in court by the end of October.

"If we decide to report articles of impeachment we could get to that in the late fall, perhaps in the latter part of the year," Nadler said Monday on MSNBC.

The Judiciary panel has also filed a petition in federal court to obtain secret grand jury material underlying Mueller's report. The request, filed on July 26, argues the panel needs the information as it weighs whether to pursue impeachment. The committee and the Justice Department agreed to file arguments by the end of September, pushing any resolution until October.

Democrats are pushing ahead with their investigations following Mueller's testimony before the Judiciary Committee in July, when he told the panel he had not "exculpated" Trump. Democrats are trying to focus public attention on Mueller's words and on his 448-page report, including several episodes in which Trump sought to influence the special counsel's investigation into Russia election interference.

Mueller concluded that he could not exonerate Trump on obstruction of justice. He also concluded there was insufficient evidence to prove a criminal conspiracy between Trump's campaign and Russia.

Nadler said the grand jury information "is critically important for our ability to examine witnesses" like McGahn and investigate the president. He said the Mueller report is not all of the evidence, but "a summary of the evidence."

Democrats are fighting the Trump administration in court on several other fronts. The House Ways and Means Committee sued the Treasury Department and IRS officials this summer in an attempt to obtain the president's tax returns. And in two other cases, the administration has tried to stop financial institutions from turning over Trump's personal records to Congress.

Trump is trying to block most every request from Congress, saying he will fight "all of the subpoenas." But Democrats think they have firm legal standing to win against him in court, and Nadler said he believes the McGahn case could break the logjam.

"Once we win that we'll win all the other subpoenas because they are basically the same legal questions," Nadler said Monday.