President Donald Trump railed furiously on Wednesday against the impeachment investigation into his dealings with Ukraine after House Democratic leaders warned the White House to expect a subpoena for documents. Democrats accused the administration of “flagrant disregard” of previous requests and said that refusal could be considered an impeachable offence. Mr Trump was also accused of “an incitement to violence” against a national security whistle-blower and advised, along with his administration, not to intimidate witnesses in the impeachment inquiry. The whistle-blower exposed a July 25 phone call between Mr Trump and Ukrainian President Volodymyr Zelenskiy in which the US president pressed for an investigation of Democratic political rival Joe Biden and his son, Hunter. Democrats say the pressure on Mr Zelenskiy on its own constitutes an abuse of power worthy of impeachment scrutiny. In the Oval Office and alongside Finland President Sauli Niinisto, Mr Trump displayed unusual anger as he defended what he has called his “perfect” phone call with Mr Zelenskiy. He said without evidence that House intelligence committee chairman Adam Schiff may have committed treason and that Mr Biden and his son were “stone-cold crooked.” At one point, Mr Trump demanded that a reporter pressing him on his dealings with Ukraine move on. “Ask the President of Finland a question, please,” he said, before calling the reporter “corrupt”. Mr Trump was asked if he would co-operate with the House to produce requested documents on the Ukrainian call. “Well, I always co-operate,” he said, although his administration has repeatedly blocked congressional investigations. “This is a hoax." Mr Schiff, accusing Mr Trump of inviting violence against the whistle-blower, had said that any effort to interfere with the Democrats’ investigations would be considered evidence of obstruction and could be included in articles of impeachment. “We’re not fooling around here,” he said. Mr Trump showed no signs of letting up, tweeting a vulgarity during the House leaders’ news conference and saying “the do-nothing Democrats should be focused on building up our country". He called Mr Schiff a “low-life” and said House Speaker Nancy Pelosi’s city of San Francisco had turned into a “tent city” of homeless.Mr Trump has tweeted in recent days that he wants to “find out about” the whistle-blower and question them, although the person’s identity is protected by the Whistleblower Protection Act. The Democrats on Friday said they would subpoena the White House for documents related to Mr Trump’s dealings with Ukraine. House oversight and reform committee chairman Elijah Cummings wrote in a memo to members that the action was necessary because the White House ignored several requests. Referring to a report on the whistle-blower’s complaint, Mr Cummings said that given the “stark and urgent warnings” the inspector general for the intelligence community has delivered to Congress, the panel has “no choice but to issue this subpoena". It will be directed at acting chief of staff Mick Mulvaney and request 13 batches of documents related to the July call and other matters. The call came as a $250 million military aid package for Ukraine was being readied by Congress but stalled by Mr Trump. White House press secretary Stephanie Grisham said the subpoena was “nothing but more document requests, wasted time and taxpayer dollars that will ultimately show the president did nothing wrong". The subpoena announcement came as House and Senate staff prepared to meet with the State Department’s inspector general on Wednesday afternoon. A State Department email said the inspector general, Steve Linick, “would like to discuss and provide staff with copies of documents related to the State Department and Ukraine". The documents were obtained from the State Department’s acting legal adviser, according to the email. Meanwhile, Secretary of State Mike Pompeo acknowledged on Wednesday that he was in on the phone call between Mr Trump and Mr Zelenskiy. Mr Pompeo also continued to object to what he said was the Democrats’ “bullying and intimidation". Democrats have scheduled closed-door depositions for Thursday with former special envoy to Ukraine Kurt Volker, and next week with removed US ambassador Marie Yovanovitch and three other State Department officials. Mr Pompeo told the committees on Tuesday that the dates they set were “not feasible”, but at least some of the officials will attend. The Democrats said that Mr Pompeo’s resistance amounted to his own intimidation. “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,” Mr Schiff, Mr Cummings and House foreign affairs committee chairman Eliot Engel said in a notice to Mr Pompeo on Tuesday. They said that if he was on Mr Trump’s call: “Secretary Pompeo 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," they said. Democrats often note that obstruction was one of the impeachment articles against Richard Nixon, who resigned the presidency in 1974 in the face of almost certain impeachment. The committees are seeking voluntary testimony from current and former officials as the House digs into State Department actions and Mr Trump’s other calls with foreign leaders, which have been shielded from scrutiny. They have also subpoenaed Mr Pompeo for documents. Mr Volker played a direct role in trying to arrange meetings between Rudy Giuliani, the president’s personal lawyer, and Mr Zelenskiy, the chairmen said. The State Department said Mr Volker confirmed that he put an adviser to Mr Zelenskiy in contact with Mr Giuliani, at that adviser’s request. The former envoy, who has since resigned his position and so is not necessarily bound by Mr Pompeo’s directions, is eager to appear as scheduled on Thursday, an insider said. Career professional Mr Volker believes he acted appropriately and wants to tell his side of the situation, the insider said. The abrupt recall from Ukraine this year of Ms Yovanovitch, a career diplomat, raised questions. The Democrats also want to hear from Ulrich Brechbuhl, a counsellor at the State Department, who also listened in on Mr Trump's call with Mr Zelenskiy. A whistle-blower alleged in an August letter to the inspector general for the intelligence community, Michael Atkinson, that the White House tried to “lock down” Mr Trump’s call because it was worried about the contents being leaked to the public. The complaint was made public after acting Director of Intelligence Joseph Maguire withheld it from Congress for weeks. It has recently been disclosed that the administration similarly tried to restrict information about Mr Trump’s calls with other foreign leaders, including Russia’s Vladimir Putin, by moving memos on to a highly classified computer system. Mr Zelenskiy said on Tuesday that he had never met or spoken with Mr Giuliani and insisted that: “It is impossible to put pressure on me." He said he stressed the importance of the military aid repeatedly in discussions with Mr Trump, but “it wasn’t explained to me” why the money didn’t come through until September. In Russia, Mr Putin said scrutiny over the phone call showed that Mr Trump’s adversaries are using “every excuse” to attack him.