The refusal of a key former White House official to testify Monday in the Democrats’ impeachment inquiry raises new questions about the pace and effectiveness of the investigation into allegations of wrongdoing swirling around President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE.

Democrats were quick to argue that the decision by Charles Kupperman, who was a deputy to former national security adviser John Bolton John BoltonJudge appears skeptical of Bolton's defense of publishing book without White House approval Maximum pressure is keeping US troops in Iraq and Syria Woodward book trails Bolton, Mary Trump in first-week sales MORE, to defy a congressional subpoena will do nothing to slow down their hard-charging probe into Trump’s dealings with Ukraine. Just hours later, Democratic leaders announced they will vote Thursday on legislation outlining the next phases of the process, to consist of public hearings as they weigh whether to introduce articles of impeachment.

Yet Kupperman’s strategy to seek court authorization before participating in the probe could prove to be a model for future witnesses, both public and private, stirring new doubts about which Trump officials will appear on Capitol Hill as the proceedings evolve.

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The wish list for Democrats features a number of other political figures — including Bolton — who may opt for Kupperman’s tactic of refusing to cooperate pending court action. Both Kupperman and Bolton share the same counsel, Charles Cooper, who declined to comment on whether Bolton will adopt the same strategy when asked to testify.

Trump’s Republican allies, who have long accused Democrats of conducting a closed-door “witch hunt,” cheered Kupperman’s strategy, framing it as the proper way to resolve the standoff between Democrats issuing witness subpoenas and a White House trying to block testimony, citing executive privilege.

“If the court says he has to come, he’s more than willing to come,” Rep. Jim Jordan James (Jim) Daniel JordanHouse panel pulls Powell into partisan battles over pandemic Sunday shows preview: Justice Ginsburg dies, sparking partisan battle over vacancy before election House passes resolution condemning anti-Asian discrimination relating to coronavirus MORE (Ohio), the senior Republican on the House Oversight and Reform Committee, told reporters in the Capitol Monday.

Rep. Adam Schiff Adam Bennett SchiffSchiff to subpoena top DHS official, alleges whistleblower deposition is being stonewalled Schiff claims DHS is blocking whistleblower's access to records before testimony GOP lawmakers distance themselves from Trump comments on transfer of power MORE (D-Calif.), who is leading the impeachment inquiry as chairman of the House Intelligence Committee, said he’s hopeful that invited witnesses will follow in the mold of the nine officials who have already testified — most of them under subpoena — in the five-week-old probe.

“I have more confidence in the witnesses that we’ve invited to appear, and were subpoenaed to appear, that they will perform as the nine witnesses that have gone before have,” Schiff told reporters in the Capitol.

Still, he threatened to charge Kupperman with contempt for defying the subpoena — an unsubtle warning shot to others who don’t comply — and acknowledged that future witnesses will be under pressure from the White House not to participate.

“It’s hard to say what other senior officials will do. I’m sure they’ll get like instructions from the White House,” Schiff said.

Democrats have made clear they’ll consider any stonewalling to be a tacit admission of presidential wrongdoing. But they’re also facing the difficult task of producing evidence damning enough to convince Trump’s GOP allies in the Senate that he’s committed impeachable offenses, an undertaking made only more difficult if witnesses — particularly political appointees close to key Ukraine events — refuse to cooperate.

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“There are other witnesses that are scheduled to come this week before us, who are going to provide hopefully even more testimony to flesh out the full extent of what happened with regard to the Ukraine, as well as who was involved and who directed it,” Rep. Raja Krishnamoorthi Subramanian (Raja) Raja KrishnamoorthiCDC causes new storm by pulling coronavirus guidance Democratic chairman says White House blocked Navarro from testifying Democrats urge CDC to update guidance to encourage colleges, universities go tobacco-free MORE (D-Ill.), a member of the Intelligence Committee, told CNN.

The president’s allies said they expect other scheduled witnesses to testify this week. That list includes career officials like Tim Morrison, who took over Fiona Hill’s role as the senior director for European and Russian affairs on the National Security Council; Kathryn Wheelbarger, acting assistant secretary of Defense for international security affairs; and State Department Ukraine experts Catherine Croft and Christopher Anderson.

“I believe they are all scheduled to appear. And obviously, we’ll be here to ask questions and get answers,” Jordan said.

As the parties sparred over the merits of Kupperman’s lawsuit, Kupperman himself weighed in, arguing it’s “reasonable” to want legal clarity.

“Given the issue of separation of powers in this matter, it would be reasonable and appropriate to expect that all parties would want judicial clarity,” he said in a statement.

Still, it’s unclear how quickly the court will rule on Kupperman’s case — an uncertainty that comes as Democrats are hoping to soon begin the public phase of their impeachment inquiry. With that in mind, Democrats said they won’t allow any legal battles to hinder their investigative efforts.

“We are not willing to allow the White House to engage us in a lengthy game of rope-a-dope in the courts, so we press forward,” Schiff said.

Kupperman’s lawsuit last week came shortly after Democrats won a resounding victory in a separate legal battle, in which a U.S. District Court judge ruled that Democrats on the House Judiciary Committee are justified in seeking to obtain grand jury materials related to former special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE’s 448-page report into Russia’s election interference and possible obstruction by Trump.

Judge Beryl Howell, an Obama appointee, pointed to Democrats’ impeachment inquiry as the justifiable reason, a ruling that further legitimized the probe into Trump’s actions amid GOP claims that the investigation was a political charade rather than a legitimate inquiry.

And Howell’s ruling, which the Justice Department says it plans to appeal, comes as Democrats are primarily focused on efforts by Trump and his associates to pressure Ukrainian President Volodymyr Zelensky into opening a corruption investigation into former Vice President Joe Biden Joe BidenFormer Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick Bloomberg rolls out M ad buy to boost Biden in Florida MORE, a leading candidate for the Democratic presidential nomination in 2020.

Democrats say other witnesses — primarily career civil servants — have backed up a government whistleblower’s charge that Trump also threatened to withhold nearly $400 million in U.S. aid to Ukraine if Zelensky failed to comply.

In their next phase, which will feature open hearings in lieu of closed-door depositions, they hope to convince the public that was the case.

“It’s a natural next step in the investigation,” Rep. David Cicilline David Nicola CicillineClark rolls out endorsements in assistant Speaker race Races heat up for House leadership posts The folly of Cicilline's 'Glass-Steagall for Tech' MORE (R.I.), the head of the Democrats’ messaging arm, said Monday. “The next phase of this is critical because we will begin really to tell the story to the American people about the president using the power of his office to pressure a foreign leader to interfere in the 2020 presidential election.”