WASHINGTON (Reuters) - Democrats in the U.S. House of Representatives who are pursuing an impeachment inquiry against President Donald Trump forged ahead with their probe on Friday, issuing a subpoena to Secretary of State Mike Pompeo for documents concerning contact with the Ukrainian government.

Following a whistleblower complaint that Trump, a Republican, solicited a political favor from Ukraine’s president that could help him get re-elected, the lawmakers are investigating concerns that Trump’s actions have jeopardized national security and the integrity of U.S. elections.

The House Foreign Affairs, Intelligence and Oversight Committees also scheduled depositions for five State Department officials over the next two weeks, including Kurt Volker, Trump’s envoy to Ukraine. Volker resigned his post on Friday, according to sources familiar with the matter.

The reason for Volker’s resignation was not immediately known. The State Department did not immediately respond to a request for comment on the subpoena or Volker’s resignation.

The committees announced the subpoena after the Trump administration missed a Thursday deadline to provide documents and information about contacts with Ukrainian officials, as well as a July 25 telephone call between Trump and Ukrainian President Volodymyr Zelenskiy.

That call is central to the impeachment investigation that Nancy Pelosi, the speaker of the Democratic-led House, announced this week.

The impeachment inquiry has cast a new pall over Trump’s presidency just months after he emerged from the shadow cast by Special Counsel Robert Mueller’s investigation into whether he colluded with Russia in the 2016 election.

Trump has reacted furiously to the impeachment inquiry, arguing he did nothing wrong, and accusing Democrats of launching a politically motivated “witch hunt.”

More than 300 former national security officials from both Republican and Democratic administrations on Friday endorsed the impeachment inquiry, saying they did not prejudge the outcome but wanted to know more facts.

TRUMP DOUBLES DOWN

As the subpoena was announced, Trump’s re-election campaign said it would spend $10 million next week airing an ad called “Biden corruption” on television and websites that accuses Democrats of playing politics with the impeachment investigation.

Former Democratic Vice President Joe Biden is Trump’s leading rival in the race for the 2020 presidential election.

The White House this week released a summary of Trump’s July 25 phone call in which he asked Zelenskiy to investigate Biden and his son Hunter Biden ahead of the November 2020 presidential election - a call at the heart of the whistleblower’s complaint.

Hunter Biden served on the board of a Ukrainian gas company called Burisma when Biden was in office. There has been no evidence that the former vice president used his position to help his son in the Ukraine matter.

Ukraine’s anti-corruption investigation agency said on Friday it was investigating permits given to companies managed by Burisma during 2010-2012. Hunter Biden was hired in 2014. The agency said it would only go further if compelling new testimony emerged.

The whistleblower raised concerns about interactions that Trump’s personal lawyer Rudolph Giuliani - who has promoted theories about Biden’s activities in Ukraine - had with former and current Ukrainian officials.

The complaint raised questions about Giuliani’s role in the early recall of Marie Yovanovitch, who was the U.S. ambassador to Ukraine. The whistleblower said Volker and Gordon Sondland, U.S. ambassador to the European Union, worked to try to “contain the damage” from Giuliani’s travels and meetings.

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The House Democratic committees said they had scheduled depositions for Yovanovitch, Volker and Sondland during the next two weeks, as well T. Ulrich Brechbuhl - a State Department aide who the whistleblower said had listened in on Trump’s call with Zelenskiy.

The committees also said it had scheduled a deposition with State Department Deputy Assistant Secretary George Kent, who oversees Ukraine policy. Kent was deputy chief of mission at the U.S. Embassy in Kiev from 2015 to 2018, and worked on anti-corruption policy in the region before that time.

House Intelligence Committee Democrats are debating the merits of summoning Giuliani, a source familiar with deliberations said.

On Oct. 4, the committee also will hear closed-door testimony from the intelligence community’s inspector general Michael Atkinson, who had determined that the whistleblower’s report was credible.

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MILITARY AID

A week before his call with Zelenskiy, Trump had ordered $400 million in military aid and foreign assistance for Ukraine to be put on hold. He lifted the freeze earlier this month.

Trump denies he pressured Zelenskiy to do anything improper and said he was not using the aid as leverage, but wanted to make sure Ukraine was taking steps to address corruption.

Defense Secretary Mark Esper said on Friday the freeze in aid to Ukraine had no impact on U.S. national security. “At this point most of the money is out the door,” Esper told reporters.

Esper said the Pentagon would provide whatever information it could to Congress about the incident. On Friday, the Pentagon’s inspector general - an internal watchdog - said it was reviewing a request from Democratic Senator Dick Durbin to investigate the hold.

Democratic leaders of two House committees asked the White House Office of Management and Budget on Friday for a long list of written details and documents by Oct. 1 about the decision to withhold the aid, saying they were concerned the delay was an abuse of executive branch authority.

A spokeswoman for the OMB said the office acted within its bounds.

“OMB has clear legal authorities to help ensure that agencies spend funds consistent with the law and the president’s priorities, and all actions taken under this administration were clearly within those authorities,” the spokeswoman, Rachel Semmel, said.

White House officials also defended the handling of records of the Trump-Zelenskiy call on Friday after the whistleblower complaint described how an electronic record had been moved from the computer server where such records are normally kept to one reserved for highly classified intelligence matters.

The Washington Post, citing three former officials, reported late Friday that records of a 2017 Oval Office conversation Trump had with two Russian diplomats - in which he told them he was unconcerned about Moscow’s meddling in the U.S. election - was limited to a few officials in an attempt to keep his comments from being disclosed publicly.

White House senior adviser Kellyanne Conway told reporters that procedures for handling records of Trump’s conversations with world leaders had changed early in his tenure.

“Early on in our time here, his calls with the leaders of Mexico and Australia leaked and then his conversation with the Russian ambassador ... leaked,” Conway said. “My understanding is we changed some of the protocols then, in other words, in handling leader calls.”