WASHINGTON — House Intelligence Committee Chairman Adam Schiff warned the White House Wednesday that "we're not fooling around here" on the impeachment inquiry as Democrats announced that they would subpoena documents related to President Donald Trump's July phone call with the leader of Ukraine.

Earlier Wednesday, Secretary of State Mike Pompeo acknowledged during a news conference in Rome that he listened to the call on which Trump pressed Ukrainian President Volodymyr Zelensky to investigate former vice president Joe Biden and his son.

Trump was tweeting as House Speaker Nancy Pelosi, D-Calif.. and Schiff held a news conference, and the president planned one of his own Wednesday afternoon.

The State Department's inspector general was also expected to meet with staff of key committees on Capitol Hill in the afternoon to share documents related to Ukraine.

Schiff, D-Calif., warned the White House that stonewalling could lead to an additional article of impeachment on obstruction of justice.

Trump asserted Wednesday that the stock market was going down because of the impeachment inquiry and accused House Democrats of trying to deliberately hurt the county.

In a pair of tweets, Trump renewed his call for Schiff to resign and attacked Democrats more broadly as "Do Nothing Democrats."

At a hearing last week, Schiff presented an embellished version of Trump's phone call with Zelensky. He later said it was meant as a parody and said that should have been apparent to Trump.

He also shared a quote from Jeanne Zaino, a political science professor at Iona College in New York state, who had appeared as a guest on Fox News.

"Nancy Pelosi and the Democrats haven't met the standards of impeachment. They have to be very careful here," read the quote.

In a memo issued Wednesday, House Oversight Committee Chairman Elijah Cummings, D-Md., said that the White House's "flagrant disregard of multiple voluntary requests for documents - combined with stark and urgent warnings from the Inspector General about the gravity of these allegations - have left us with no choice but to issue this subpoena."

The subpoena will be issued Friday, according to Cummings's memo.

The memo said the subpoena will seek documents that the committee first requested on Sept. 9.

In response to a multipart question, he did not say whether he was comfortable with Trump's pressing of Zelensky to investigate Biden and his son, Hunter.

Pompeo said the call focused on issues such as the threat that Russia poses to Ukraine and the need for Ukraine to root out corruption.

He said the United States would consider to pursue those issues "even while all this noise is going on."

During a Sept. 22 appearance on ABC News's "This Week," Pompeo was asked what he knew about Trump's conversation with Zelensky following an initial Wall Street Journal report that the call was part of a whistleblower complaint.

Pompeo responded by saying he hadn't seen the whistleblower report. He later said he had seen a statement from the Ukrainian foreign minister that there was no pressure applied on Zelensky. Pompeo made no mention of being on the call.

During his news conference Wednesday, Pompeo also repeated his claims from a letter on Tuesday that House Democratic staffers have been seeking to intimidate State Department officials in their efforts to learn more about Trump's call with Zelensky.

"We won't tolerate folks on Capitol Hill bullying, intimidating State Department employees. That's unacceptable," Pompeo said.

Former staff members who worked on foreign leader calls said it is very unusual for a secretary of state to listen in on calls with leaders from a country as small as Ukraine.

Partly it is because the secretary of state's schedule is very busy and rarely aligns with the president's schedule of routine calls to heads of state, so they arrange only to be on major foreign leader conversations.

When Rex Tillerson was secretary of state, for example, he would coordinate plans to listen in on Trump's calls with Russian President Vladimir Putin and German Chancellor Angela Merkel.

The former staffers on the National Security Council said Pompeo's presence on this call suggests the subject or the purpose of the call had high importance to the president, and thus to him. The former staffers spoke on the condition of anonymity to speak more candidly.

Also, Wednesday, Steve Linick, the State Department's inspector general, plans to meet with staffers of key House and Senate committees Wednesday at 3 p.m. at his request.

The committees were notified Tuesday that Linick wants "to discuss and provide staff with copies of documents related to the State Department and Ukraine," according to a letter obtained by The Washington Post.

The offer by Linick's office, which operates mostly independently from the State Department and is responsible for investigating abuse and mismanagement, comes amid a standoff between Pompeo and House Democrats, who are demanding documents and testimony on Ukraine-related matters for their impeachment inquiry.

Linick's office "obtained the documents from the acting legal adviser of the Department of State," the letter said. The inspector general doesn't have to seek Pompeo's approval to approach Congress with information, especially if it is not classified.

It is unclear exactly what Linick will provide the committees, which include the panels in charge of foreign relations, intelligence, appropriations and oversight in the House and Senate. But the demand for any credible information related to Ukraine and the State Department is at a fever pitch as Democrats seek to build the case for Trump's ouster based on his dealings with Ukraine's leadership.

This article was written by John Wagner, a reporter for The Washington Post.