WASHINGTON — The man whose testimony could ultimately prove crucial in a potential impeachment battle isn’t appearing before a congressional committee this week. The biggest question facing Democrats is whether he will ever testify at all.

Even as House investigators prepare for Wednesday’s much-anticipated appearance of former special counsel Robert Mueller in front of two House committees, Judiciary Chairman Jerry Nadler is readying his next step: seeking to compel testimony from former White House counsel Don McGahn, the person considered to be the potential linchpin of an obstruction of justice case against President Donald Trump.

Few names appear more often — more than 500 times in the redacted version of Mueller’s final report — than McGahn’s. The longtime Republican insider who served as both Trump’s campaign counsel and first White House counsel sat for more than 30 hours of voluntary testimony before Mueller’s team.

McGahn is a key player in what Democrats believe is one of the clearest cases of obstruction of justice outlined in the report: Trump’s directive that he fire Mueller. Trump has disputed McGahn’s account to Mueller since the report was published, placing greater import on what McGahn would testify to publicly under oath before Congress.

The committee could move within days to request a court order to enforce its subpoena for documents and in-person testimony from McGahn, multiple officials tell NBC News. And staff members preparing lawmakers for the Mueller hearing say you can expect to hear a lot about him Wednesday, as Democrats seek to bolster their argument for piercing the White House’s claim that McGahn is barred from testifying because of executive privilege concerns.

“Mueller is an Act 1 witness. McGahn is Act 2,” said Rep. Eric Swalwell, D-Calif., a member of both committees questioning Mueller on Wednesday. “Mueller lays out who we need to hear from. I expect hearing from McGahn directly will be a priority for the Judiciary Committee following Mueller.”

So-called fact witnesses like McGahn are critical to the Judiciary Committee, especially given members’ expectation that Mueller will be reluctant to go beyond what is included in the report he finished in April.

“Mueller is not a fact witness but he does have access to facts that we don’t have, so that’s helpful. McGahn is an actual fact witness,” said Rep. Zoe Lofgren, D-Calif., a senior member of the Judiciary Committee. “We want to see the fact witnesses and we think we have a right to see the fact witnesses.”

It was McGahn’s refusal — on orders from the White House — not to testify before the Judiciary Committee on May 21 that helped escalate calls among Democrats to launch a formal impeachment inquiry.

The committee’s inability to bring him in so far is at the heart of one of the key debates within the Democratic caucus, and an inherent contradiction in their approach so far. Some Democrats believe testimony from McGahn that expands upon the Mueller report’s findings could give them their best ammunition legally and politically for launching the impeachment inquiry.

But others believe the courts are only likely to expedite forcing McGahn to testify if an impeachment process is already underway — a case that some top committee Democrats have already made to House Speaker Nancy Pelosi. Absent that, past civil contempt battles between the two branches of government have taken years to play out.

“As important as it is that the American public hear from Mueller, he’s just a prosecutor, not a witness to the president’s misconduct,” said Matthew Miller, a former Justice Department spokesperson and an NBC News legal analyst. “The key witness remains Don McGahn, and if the House wants to have blockbuster hearings that show the American people how the president has obstructed justice, they need to get McGahn in the witness seat sooner rather than later.”

The White House has cited Justice Department guidance in arguing that McGahn is “absolutely immune from compelled congressional testimony with respect to matters occurring during his service as a senior adviser to the president,” as White House counsel Pat Cipollone told the committee in May.

McGahn’s attorney said at the time that he was “obligated to maintain the status quo and respect the president’s instruction,” and that “the committee’s dispute is not with Mr. McGahn but with the White House.”

The full House voted June 11 to authorize the Judiciary Committee to go to court to enforce its subpoena, authority the panel has yet to use. Nadler said in an interview with “Fox News Sunday” that one reason for the delay was that the House’s top lawyer has been “so busy enforcing subpoenas — other subpoenas.”

“It is our duty to make sure that people like McGahn testify," Nadler said, "because as the Mueller report showed, the president asked him to fire the special counsel so as to stop the investigation into the president.”

The Judiciary Committee had reached an agreement with McGahn’s former chief of staff in the White House, Annie Donaldson, to provide written responses to questions. Democrats say that White House lawyers blocked her from doing so in 212 instances, according to a transcript, including whether she was present when Trump made certain directives including urging Attorney General Jeff Sessions not to recuse himself from the Russia probe.

Rep. Doug Collins, R-Ga., the top Republican on the Judiciary Committee, cast the focus on McGahn as a sign of desperation from Democrats who fear Mueller's appearance will not yield any new information.

“They're already trying to move from this because they realize that they're probably not going to get anything that feeds the base that they're trying to feed,” Collins said. “If I was them, I’d want to distract from this too, because they've sold a bill against the American people that they can't deliver.”