Rep. Jerry Nadler, chairman of the House Judiciary Committee, accused President Donald Trump of a "shocking and dangerous" abuse of executive power Monday night after the White House instructed former aides Rob Porter and Rick Dearborn to defy the panel's subpoenas to testify as part of its investigation into Trump's alleged crimes.

In a letter (pdf) to Nadler on Monday, White House counsel Pat Cipollone said Porter and Dearborn are "absolutely immune" from testifying and that Trump has ordered them not to do so.

"If he were to prevail in this cover-up while the Judiciary Committee is considering whether to recommend articles of impeachment, he would upend the separation of powers as envisioned by our founders."

—Rep. Jerry Nadler

Nadler slammed the Trump White House's "assertion of executive privilege and absolute immunity" in a statement responding to the letter.

"The president would have us believe that he can willfully engage in criminal activity and prevent witnesses from testifying before Congress," Nadler said. "If he were to prevail in this cover-up while the Judiciary Committee is considering whether to recommend articles of impeachment, he would upend the separation of powers as envisioned by our founders."

The New York Democrat also highlighted the White House's efforts to limit former Trump campaign manager Corey Lewandowski's testimony, which is scheduled for Tuesday. The Guardian reported that the president instructed Lewandowski, who never worked for the White House, to not reveal private conversations with Trump beyond what is in the former special counsel Robert Mueller's report.

"No one is above the law," said Nadler. "The House Judiciary Committee will continue our investigation of the president's crimes, corruption and cover-up and get to the truth for the American people."

Nadler's statement came hours before the first hearing of what some House Democrats are describing as an impeachment inquiry. The committee voted last week to outline the terms of "how the party's ongoing investigations into alleged presidential misconduct could lead to drafting—and eventual votes on—impeachment articles," The Hill reported.

Progressives remain concerned that the Judiciary Committee is still not doing enough to hold Trump accountable.

In response to the committee's vote last week, a coalition of progressive groups including Credo Action and Democracy for America said the panel is "refusing to set a date to move articles of impeachment out of committee or to officially expand the scope of its inquiry beyond the Mueller report, despite overwhelming evidence that the president has committed multiple impeachable offenses."

"The stakes are too high and the hour is too late for Congress to extend this process any longer than that timeframe," the groups said. "Our Constitution, our democracy, our communities, and our planet's future are on the line. The House majority must act with the urgency required in the face of this moral and constitutional crisis."