WASHINGTON — President Donald Trump, desperate to undermine a fast-moving impeachment probe, could be adding to his troubles.

House Democrats say Trump’s stonewalling and threats to unmask a whistleblower could lead to obstruction charges if articles of impeachment are drafted over the president’s request for Ukraine’s government to investigate a potential Democratic opponent in next year’s election.

“Congress will stand up to the massive obstruction of the Trump administration if they continue down this course,” Rep. Ted Lieu, D-Calif., a member of the House Judiciary Committee, said Tuesday.

Trump’s allies made clear they’re digging in their heels, however, resisting the Democrats’ demands for documents and testimony.

Secretary of State Mike Pompeo said five current and former State Department officials who dealt with Ukraine would not provide depositions to House investigators as scheduled because they had “woefully inadequate” time to prepare, and because the request was not made through normal channels.

In a sharply worded three-page letter to Rep. Eliot Engel, chairman of the House Foreign Affairs Committee, Pompeo said the Democrats’ request “can be understood only as an attempt to intimidate, bully, and treat improperly” State Department professionals, including members of the foreign service.

Some Trump supporters cheered Pompeo’s response to the Democrats.

But it also complicated Pompeo’s own situation, coming a day after it was disclosed that he had listened in during Trump’s July 25 phone call with Ukrainian President Volodymyr Zelenskiy that helped trigger the impeachment inquiry.

On Wednesday, the State Department’s inspector general is expected to brief congressional staff from several House and Senate appropriations, oversight, foreign affairs and intelligence committees on their requests for information and documents on Ukraine, according to an aide familiar with the planning. The inspector general acts independently from Pompeo.

The committees are seeking voluntary testimony from current and former officials as the House digs into State Department actions and Trump’s other calls with foreign leaders that have been shielded.

Kurt Volker, the former U.S. special representative to Ukraine, is on track to give his closed-door deposition Thursday. Volker resigned last week after a whistleblower complaint detailed Trump’s interactions with Zelenskiy.

Marie Yovanovitch, who was U.S. ambassador to Ukraine before she was recalled in May, will give her deposition Oct. 11.

Democrats responded to Pompeo’s letter, saying he should “immediately cease intimidating (witnesses) to protect himself and the President.”

“Any effort to intimidate witnesses or prevent them from talking with Congress — including State Department employees — is illegal and will constitute evidence of obstruction of the impeachment inquiry,” said three House chairmen, Adam Schiff, D-Calif., of the intelligence committee, Engel, D-N.Y., of Foreign Affairs; and Elijah Cummings, D-Md., of Oversight.

In halting any appearances by State officials, and demanding that executive branch lawyers accompany them, Pompeo is underscoring Attorney General William Barr’s view of White House authority.

“I will use all means at my disposal to prevent and expose any attempts to intimidate the dedicated professionals,” Pompeo wrote.

When issuing a separate subpoena last week as part of the inquiry, the chairmen of the three House committees made it clear that stonewalling their investigation would be fought.

“Your failure or refusal to comply with the subpoena shall constitute evidence of obstruction of the House’s impeachment inquiry,” the chairmen wrote.

It’s unclear whether Pompeo will comply with the committees’ request for documents by Friday. He had declined to comply with their previous requests for information.

Pompeo, in Italy to meet with the country’s president and prime minister, ignored shouted question about the impeachment inquiry Tuesday.

Volker played a direct role in arranging meetings between Rudy Giuliani, who is Trump’s personal lawyer, and Zelenskiy, the chairmen said.

The State Department said that Volker has confirmed that he put a Zelenskiy adviser in contact with Giuliani, at the Ukraine adviser’s request.

Giuliani suggested he might defy a House subpoena for documents regarding his meetings with Ukrainian officials, saying it “raises significant issues concerning legitimacy and constitutional and legal issues,” including attorney-client privilege.

On Tuesday, Giuliani hired former federal prosecutor Jon Sale to represent him in the widening inquiry.

It’s unclear whether House Speaker Nancy Pelosi, D-Calif., would allow the potential case to expand to include obstruction. She wants Democrats to focus on Trump’s alleged improper dealings with Ukraine, which she says betrayed his oath of office and jeopardized national security.

At the center of the inquiry is Trump’s phone call with Zelenskiy. When Zelenskiy brought up U.S. military aid, Trump immediately asked for his help with two investigations.

Trump urged Zelenskiy to “look into” former Vice President Joe Biden, a leading Democratic presidential candidate, and his son Hunter, who had worked for a Ukrainian natural gas company.

Trump also asked Zelenskiy about CrowdStrike, a cybersecurity firm that established Russians had hacked the Democratic National Committee computer networks during the 2016 campaign. U.S. intelligence agencies and the Justice Department backed up that conclusion, but Trump has repeatedly questioned it.

Over the past week, Trump has repeatedly lashed out at an anonymous whistleblower, whose complaint about the call first alleged that Trump was using the power of his office to solicit interference from a foreign government in the 2020 election.

One senior Republican offered support for the whistleblower Tuesday, despite the president’s attacks.

“This person appears to have followed the whistleblower protection laws and ought to be heard out and protected,” Sen. Chuck Grassley, R-Iowa, who worked on laws to protect government employees who report wrongdoing, said in a statement. “We should always work to respect whistleblowers’ requests for confidentiality.”

Jay Sekulow, one of the president’s personal lawyers, said Trump has done nothing wrong by complaining about what he considers a partisan witch hunt.

“The president is allowed to exercise his First Amendment rights to defend himself,” he said.

Some outside experts urged Congress to add Trump’s attacks and his stonewalling to any impeachment charges.

“An obstruction charge would be an important part of setting a standard for how presidents should behave,” said John Hudak, a senior fellow at the nonpartisan Brookings Institution. “There is significant evidence that would suggest Congress should look into obstruction in their inquiry.”

David Iglesias, a former U.S. attorney who directs the Wheaton Center for Faith, Politics and Economics in Illinois, called Trump’s threats to unmask the whistleblower “grossly inappropriate” but said adding them to articles of impeachment could muddy a straightforward case.

“You should always stick to the most readily provable offense,” Iglesias said.

Associated Press contributed.