On Tuesday, the secretary of state, Mike Pompeo, dismissed summons from Democratic committee chairmen in the House of Representatives for five current and former state department officials to testify on the president’s attempts to push Ukraine to dig up dirt on his leading political rival.

Pompeo wrote to the House foreign affairs committee, accusing Democrats of “an attempt to intimidate, bully, and treat improperly” the officials. The requested dates were “not feasible”, he said.

The speed of the investigation raises “significant legal and procedural concerns”, Pompeo wrote. “I will not tolerate such tactics, and I will use all means at my disposal to prevent and expose any attempts to intimidate the dedicated professionals.”

However, it soon became clear that Pompeo had only limited power to stop the congressional committees from gathering evidence for an impeachment inquiry. One of the five witnesses deposed, Kurt Volker, former special envoy for Ukraine who resigned last week, confirmed he would speak to the committees in closed session on Thursday, according to the Associated Press.

House sources were quoted as saying that a second witness, Marie Yovanovitch, former ambassador to Kyiv, would appear on 11 October, nine days later than originally scheduled.

And perhaps most damaging of all, the state department’s inspector general, Steve Linick, who acts as an independent watchdog, informed the House committees that he wanted to brief them on Wednesday on documents concerning relations with Ukraine that had been obtained from the department’s legal adviser.

It was unclear whether Pompeo had been warned of Linick’s move.

Since Democrats announced the impeachment inquiry last week, the foreign affairs, intelligence and oversight committees have wasted little time in seeking documents and testimonies.

The committee chairs, Eliot Engel, Adam Schiff, and Elijah Cummings, dismissed Pompeo’s criticisms.

“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,” they said in a joint statement.

“In response, Congress may infer from this obstruction that any withheld documents and testimony would reveal information that corroborates the whistleblower complaint.”

 This is the same Pompeo who removed a career ambassador in Ukraine, and stood by as she was subject to smears

Pompeo was assailed on Tuesday by former officials who saw hypocrisy in his expressions of concern over intimidation of foreign service officers. As a congressman, Pompeo regularly cross-examined diplomats over the 2012 attack on the US consulate in Benghazi, Libya, in an attempt to show that Hillary Clinton had been negligent as secretary of state.

State department employees are currently being subjected to questioning over emails they sent to Clinton’s private server when she was in office. According to the Washington Post, the employees have been informed that the emails have been classified retroactively and therefore could represent security violations.

Jeffrey Prescott, a former special White House assistant to Barack Obama, said on Twitter: “This is the same Pompeo who removed a career ambassador from her post in Ukraine, and stood by as she was subject to smears.”

The White House has stonewalled numerous congressional investigations. Last month it ordered Rob Porter, the ex-White House staff secretary, and Rick Dearborn, who was deputy chief of staff, to defy subpoenas regarding special counsel Robert Mueller’s Russia investigation. Analysts suggest Trump sees little downside in brazen displays of impunity.

The president and his allies have attacked the House impeachment inquiry relentlessly, while ramping up spending on ads for Trump’s re-election campaign. In a series of tweets late Tuesday evening, Trump said efforts by Democrats amounted to “a COUP”.

In issuing a separate subpoena last week as part of the impeachment inquiry, the chairmen of three committees made it clear stonewalling would be considered obstruction of Congress.

The expanding Ukraine scandal is threatening to engulf one of the president’s most loyal enforcers. Pompeo received a subpoena from the committees to turn over documents related to the Ukraine investigation. He said he would respond by the deadline of 4 October.

It also emerged this week that Pompeo participated in the July phone call in which Trump, having frozen military aid, pressed the Ukrainian president, Volodymyr Zelenskiy, to investigate baseless allegations against the former vice-president Joe Biden and his son Hunter.

The three committee chairs said on Tuesday that if the report of Pompeo being on the call were true, he “is now a fact witness in the House impeachment inquiry. He should immediately cease intimidating department witnesses in order to protect himself and the president.”

The secretary of state began a four-nation tour of Europe on Tuesday in Italy. He was accompanied by the former White House adviser Sebastian Gorka, who has a radio show and is travelling as a member of the press. Gorka is a hardline nationalist and former editor for the far-right Breitbart News. He has vehemently denounced the impeachment inquiry.

Trump is also defying political norms by accusing opponents of treason and making threats to the intelligence community whistleblower who raised concerns about the Zelenskiy call.

In an earlier round of volatile tweets on Tuesday, Trump had wondered why he was not “entitled to interview [and] learn everything about” the whistleblower, whose identity is protected by law.

The president also repeated his claim that the July call was “PERFECT”, adding: “This is just another Fake News Media, together with their partner, the Democrat Party, HOAX!”

But the Republican senator Chuck Grassley told reporters the whistleblower “ought to be heard out and protected” and requests for confidentiality should be respected.

The crisis has also cast a harsh spotlight on the attorney general, William Barr. The Washington Post revealed that he held private meetings overseas with foreign intelligence officials, seeking their help in a justice department investigation Trump hopes will undermine his own intelligence agencies’ conclusions about Russian interference in the 2016 election.

The countries concerned were Britain, Australia and Italy.

The Republican senator Lindsey Graham defended the strategy.

“Barr should be talking to Australia,” he told the Fox News host Sean Hannity. “He should be talking to Italy. He should be talking to the UK to find out if their intelligence services worked with our intelligence services improperly to open up a counter-intelligence investigation of Trump’s campaign.

“If he’s not doing that, he’s not doing his job. So I’m going to write a letter to all three countries … asking them to cooperate with Barr.”

Richard Painter, a former chief White House ethics lawyer under George W Bush, tweeted: “Impeach Barr and Pompeo after impeaching ⁦@realDonaldTrump⁩.”

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