Democrats have issued a subpoena to the White House, demanding it hand over documents relating to Donald Trump’s controversial Ukraine phone call as they sharply turned up the pressure of their impeachment probe.

Two days after Democrats warned the president they would issue the subpoena if he did not cooperate, and just hours after he admitted his opponents had the numbers in the House to “easily” impeach him, a letter was sent to White House stating that it was expected to hand over the materials being sought.

In a letter sent on Friday afternoon, the Democratic chairs of three House committees – Adam Schiff, Eliot Engel and Elijah Cummings – wrote that they had no choice but to take the steps they were embarking upon.

“We deeply regret that President Trump has put us – and the nation – in this position, but his actions have left us with no choice but to issue this subpoena,” they said.

“The White House has refused to engage with – or even respond to – multiple requests for documents from our committees on a voluntary basis. After nearly a month of stonewalling, it appears clear that the president has chosen the path of defiance, obstruction, and cover-up.”

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Earlier in the day, Mr Trump was asked whether he would provide Democrats the documents they sought before the Friday deadline expired.

He replied: “I don’t know. That’s up to the lawyers. I know the lawyers think they’ve never seen anything so unfair. They’ve never seen anything so unjust.

“I’ve been president now for almost three years, and I’ve been going through this for almost three years. It’s almost become, like, a part of my day.”

He admitted that Democrats in the House of Representatives had the numbers to impeach him if they wished to, though he said he believed Republicans in the Senate would not confirm the censure. “So the Democrats, unfortunately they have the votes,” he said.

After the subpoena was filed, the White House issued a statement claiming that it made little difference.

“This subpoena changes nothing – just more document requests, wasted time, and taxpayer dollars that will ultimately show the president did nothing wrong,” said spokesperson Stephanie Grisham.

“The do-nothing Democrats can continue with their kangaroo court while the president and his administration will continue to work on behalf of the American people.”

Trump accuses Adam Schiff of lying to Congress over Ukraine call

While the White House sought to brush off the moves, it seems clear Mr Trump realises there is little he can do to avoid being impeached, if House speaker Nancy Pelosi seeks to call for a vote.

While she has long been cautious about pursuing such a route, she has been left with few options after the outrage triggered by claims of an intelligence community whistleblower that Mr Trump sought to pressure the leader of Ukraine to dig up dirt on Joe Biden.

If she does not push for a vote of the House, it is hard to see how long she would retain her position as the Democrats’ leader in the lower chamber.

Reports said the White House was expected to send a letter to Ms Pelosi arguing that congress could not mount its impeachment inquiry without first having a vote to authorise it. The letter was expected to say the administration would not cooperate with the probe without that vote.

Mr Trump said the resolution would likely pass the House, but he predicted it would backfire on Democrats. “I really believe that they’re going to pay a tremendous price at the polls,” he said.

Mr Trump’s comments came shortly before Democrats sent an extensive request for documents to vice president Mike Pence about his contacts with Ukraine. Mr Pence spokesperson Katie Waldman, dismissed the new demand for documents, saying that given its wide scope, “it does not appear to be a serious request”.

It is likely the White House will claim the material sought by congress is covered by executive privilege, an issue that will likely be resolved by the courts.

What is also unclear is what impact the impeachment battle will have on the 2020 election campaign. Polls show the country is roughly divided on whether Democrats should press ahead with such a move.

The president believes the move could help energise his base, as he decries the Democrats’ actions as a “witch hunt”.