US PRESIDENT DONALD Trump has lashed out against the investigation into his dealings with Ukraine, hours 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.

Trump fired back saying that Democrats should be “focused on building up our Country, not wasting everyone’s time and energy on BULLSHIT, which is what they have been doing ever since I got overwhelmingly elected in 2016,” he tweeted.

Separately, the Democrats accused President Trump of “an incitement to violence” against a national security whistleblower and advised him and his administration not to intimidate potential witnesses in the impeachment inquiry.

The whistleblower exposed a July phone call that President Trump had with Ukrainian President Volodymyr Zelenskiy in which he pressed for an investigation of Democratic political rival Joe Biden and his family.

Democrats say the pressure on Zelenskiy, on its own, constitutes an abuse of power worthy of impeachment scrutiny.

The Do Nothing Democrats should be focused on building up our Country, not wasting everyone’s time and energy on BULLSHIT, which is what they have been doing ever since I got overwhelmingly elected in 2016, 223-306. Get a better candidate this time, you’ll need it!

— Donald J. Trump (@realDonaldTrump) October 2, 2019

In appearances in the Oval Office and a joint press conference with the president of Finland, Trump displayed an unusual show of anger as he defended what he has called his “perfect” phone call with Zelenskiy.

He suggested, without evidence, that House intelligence committee Chairman Adam Schiff may have committed treason, and, again without evidence, labelled Biden and his son “stone cold crooked”.

At one point, President Trump demanded that a reporter pressing him on his dealings with Ukraine move on.

“Ask the president of Finland a question, please,” he said, emphasising each word, eventually labelling the reporter “corrupt”.

Trump declined to answer yes or no when asked if he would cooperate with the House to produce requested documents on Ukraine.

“Well, I always cooperate,” he said, though his administration has repeatedly stonewalled congressional investigations. “This is a hoax,” he added.

Schiff, accusing President Trump of inviting violence against the whistleblower, had said earlier 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.

Congressman Adam Schiff should resign for the Crime of, after reading a transcript of my conversation with the President of Ukraine (it was perfect), fraudulently fabricating a statement of the President of the United States and reading it to Congress, as though mine! He is sick!

— Donald J. Trump (@realDonaldTrump) October 2, 2019

Trump has tweeted in recent days that he wants to “find out about” the whistleblower and question him or her, though the person’s identity is protected by the Whistleblower Protection Act.

The Democrats said they would subpoena the White House tomorrow for documents related to President Trump’s dealings with Ukraine.

House Oversight and Reform Chairman Elijah Cummings wrote in a memo to committee members that the action is necessary because the White House has ignored multiple requests.