U.S. Secretary of State Mike Pompeo arrives to an event hosted by the Department of State's Energy Resources Governance Initiative in New York, Thursday, Sept. 26, 2019. Credit: Seth Wenig | AP

WASHINGTON — House Democrats said they will ramp up their impeachment effort next week against President Donald Trump, and on Friday the chairmen of three powerful House committees subpoenaed Secretary of State Mike Pompeo for documents related to their investigation into whether the president improperly pressured Ukraine’s leader to investigate his political rivals.

Members of the House Intelligence Committee have pledged to stay in Washington to work through Congress’s scheduled recess next week as Democrats seek to build a case for removing the president from office.

“I can tell you it’s going to be a very busy couple of weeks ahead,” Intelligence Committee Chairman Adam Schiff, D-Calif., told reporters. “We’re going to be trying to schedule hearings, witness interviews. We’ll be working on subpoenas and document requests. We’ll be busy.”

The House Intelligence, Oversight and Foreign Affairs panels also announced Friday that they have scheduled five depositions beginning with State Department officials who would have knowledge of Trump’s engagements with Ukraine.

Democrats are working against an unofficial deadline of the end of the year to complete impeachment proceedings that currently focus on Trump’s request for “a favor” during a July telephone call with Ukrainian President Volodymyr Zelensky.

“The speaker has made it very clear that we are not to let momentum drop in these two weeks,” said Rep. Jim Himes, D-Conn.

Democrats are treating a whistleblower complaint surrounding Trump’s call with Zelensky as a “road map” for their probe, looking to the account of how White House officials were alarmed by the call and allegedly sought to improperly keep its contents secret.

At issue is Trump’s pressing Zelensky on the call to investigate former vice president Joe Biden, a leading 2020 Democratic presidential contender, and his son Hunter as well as an unsubstantiated theory that Ukrainians worked with Democrats to interfere in the 2016 election.

Hunter Biden served for nearly five years on the board of Burisma, Ukraine’s largest private gas company, whose owner came under scrutiny by Ukrainian prosecutors for possible abuse of power and unlawful enrichment. Hunter Biden was not accused of any wrongdoing in the investigation. As vice president, Joe Biden pressured Ukraine to fire the top prosecutor, Viktor Shokin, who Biden and other Western officials said was not sufficiently pursuing corruption cases. At the time, the investigation into Burisma was dormant, according to former Ukrainian and U.S. officials.

Trump spent Friday railing against the impeachment effort as a dangerous partisan “Witch Hunt!” while maintaining that he had said nothing wrong in his call with Zelensky.

“If that perfect phone call with the President of Ukraine Isn’t considered appropriate, then no future President can EVER again speak to another foreign leader!” he tweeted.

The president and his allies also continued to try to undermine the credibility of the whistleblower, whose identity has not been publicly released.

Trump speculated Friday morning on Twitter that the whistleblower might have received information from “a leaker or spy” or a “partisan operative.” He offered no evidence for his suggestions.

Trump’s reelection campaign announced Friday that it will launch a television ad “highlighting Joe Biden’s Ukraine scandal and the Democrats’ plan to use it to steal the 2020 election by impeaching the President.”

Republicans have mostly lined up to support Trump, echoing his claim that the call was “perfect” and that his willingness to release both a transcript of it and a version of the whistleblower complaint serve to show he has nothing to hide.

But there is unease, as suggested by a previously unreported intercession by Senate Majority Leader Mitch McConnell, R-Ky.

Two people familiar with the conversation said McConnell told the White House earlier this week that Trump needed to release the transcript of his call to bolster the claim that the conversation was not improper because the speculation about what happened was becoming politically untenable.

Neither the White House nor McConnell’s office provided comment on that communication.

Democrats’ inquiry, though on a tight schedule, is potentially broad. The whistleblower’s letter indicates that approximately a dozen White House officials listened in on the July call between Trump and Zelensky, and that “multiple” intelligence and diplomatic officials were briefed on its contents.

The complaint also highlights the role played by Trump’s lawyer, Rudy Giuliani, and the State Department officials who reportedly advised the Ukrainians on how to “navigate” Trump’s demands.

Lawmakers are seeking to depose or interview several of those officials and are also interested in learning what other transcripts of Trump’s communications might have been moved to a secure server for political protection, as the whistleblower alleges.

The subpoena issued Friday demands that Pompeo turn over by Oct. 4 documents pertaining to Trump’s dealings with Zelensky.

“Your failure or refusal to comply with the subpoena shall constitute evidence of obstruction of the House’s impeachment inquiry,” Foreign Affairs Chairman Eliot Engel, D-N.Y., Intelligence Committee Chairman Schiff and Oversight Committee Chairman Elijah Cummings, D-Md., wrote in a joint letter to Pompeo.

In addition, the House Intelligence Committee will hold a closed briefing with the inspector general of the intelligence community Oct. 4. The IG conducted a preliminary investigation and determined that the whistleblower complaint was credible.

“We will move as expeditiously as possible, but we have to see what witnesses are going to make themselves available and what witnesses are going to require compulsion,” Schiff said Friday.

Among the State Department officials the committee is seeking to depose are Kurt Volker, a former ambassador who has worked for Pompeo as an envoy on Ukraine issues, and Marie Yovanovitch, a Foreign Service officer who this year was recalled early from her post as ambassador to Ukraine and has been criticized by Trump. Volker resigned from his State Department post Friday, according to several media reports.

The move by the House committee chairmen brought the State Department fully within Congress’s sights in the impeachment drama, a place that Pompeo has assiduously tried this week to avoid. In public appearances at the United Nations General Assembly, which he attended with Trump, Pompeo brushed aside reporters’ questions on the role the State Department played in the Ukraine affair. He is to leave Monday for a weeklong diplomatic trip to Europe.

The committees demanded Pompeo’s appearance on Capitol Hill and listed questions they want answered about his knowledge of the activities of Giuliani, Trump’s personal attorney, and whether the State Department assisted Giuliani in any way — as Giuliani has asserted — in pushing Trump’s demands with Ukrainian officials.

The chairmen also asked about the activities of Volker and Gordon Sondland, the U.S. ambassador to the European Union, both of whom met with Zelensky the day after the July telephone call.

Rep. Jackie Speier, D-Calif., another member of the Intelligence Committee, said next steps will include speaking with the whistleblower, officials who listened in on the phone call and Giuliani.

“I think that we are going to want to interview people that were engaged in what is clearly a scheme. This is not a one-off. This is not just a phone call. This is a scheme that was hatched some time ago, and you can see evidence of it dating back to 2018,” Speier said. “And I want to see us come up with that timeline, because this is a truly corrupt undertaking.”

Separately, two powerful committees that oversee the federal budget and spending have requested from the White House a timeline and relevant documents that explain how the United States came to withhold almost $4oo million of military aid to Ukraine.

Central to the committees’ concerns are reports that the White House Office of Management and Budget was responsible for relaying to the State Department and the Pentagon the president’s order to hold back the aid to Ukraine.

“As reports continue to emerge, we have deepening concerns that OMB continues to demonstrate a pattern of impeding agencies’ ability to use their enacted appropriations,” wrote House Appropriations Chairwoman Nita Lowey, D-N.Y., and House Budget Chairman John Yarmuth, D-Ky., in a joint letter to acting White House chief of staff Mick Mulvaney and OMB acting director Russell Vought.

Washington Post writers Karen DeYoung, Colby Itkowitz and John Wagner contributed to this report.