The Trump administration complained on Saturday of harassment by Democratic lawmakers who issued subpoenas for White House documents regarding President Donald Trump's alleged pressuring of Ukraine for political favours.
And in a key development, a potential second whistleblower was reportedly weighing up whether to come forward.
The latest explosive turns in the impeachment investigation into Trump came as Secretary of State Mike Pompeo accused Democratic-led congressional committees leading the inquiry of having "harassed and abused" State Department employees by contacting them directly for documents rather than going through department lawyers.
"That's harassment," Pompeo said during a visit to Athens, "and I'm never going to let that happen to my team."
The House committees leading the probe issued their subpoenas as evidence mounted that Trump abused his powers by attempting to use US military aid to pressure Ukrainian President Volodymyr Zelensky to seek damaging information on 2020 Democratic rival Joe Biden.
The investigations were touched off by reports that an original whistleblower - reportedly a CIA analyst - had filed a formal complaint to the intelligence community inspector general about Trump's alleged pressuring of Zelensky.
Trump and his supporters have aggressively attacked that still-anonymous whistleblower. Trump denounced the person as "close to a spy" and a Democrat operating with second-hand information.
In a tweet early Saturday, Trump said that "the so-called Whistleblower's account of my perfect phone call is 'way off,' not even close... This is a fraud against the American people!"
But if a second whistleblower emerges, and if that person in fact has more direct information about the events in question, as the New York Times reported late Friday, the allegations would become more difficult to shrug off.
The Times said the second official was among those interviewed by inspector general Michael Atkinson about the allegations lodged by the original whistleblower.
The newspaper, citing two people briefed on the matter, said the second official was still pondering whether to lodge a formal complaint.
Congressional investigators, led by House Intelligence Committee chairperson Adam Schiff, have promised a quick and expeditious inquiry, but said on Friday that the White House was not cooperating.
"The White House has refused to engage with - or even respond to - multiple requests for documents," said a letter signed by the Democratic chair of the House oversight, intelligence and foreign affairs committees.
"His actions have left us with no choice but to issue this subpoena."
In their letter to acting White House Chief of Staff Mick Mulvaney, the committees demanded that he turn over the requested files by October 18.
The subpoena followed a demand earlier on Friday for documents from Vice President Mike Pence.
Pompeo - who recently acknowledged having listened in on the key Trump-Zelensky phone call - failed to meet a Friday subpoena deadline to turn over Ukraine-related documents, CNN reported, citing a House Foreign Affairs committee aide.
Meanwhile, a series of text messages between US diplomats dealing with Ukraine, made public by the congressional investigators, supported Democratic accusations that Trump had illegally sought foreign help for his reelection effort.
'Most corrupt president we've had in modern history'
Trump pushed back hard, saying there was no quid pro quo and, in an effort to recast the entire saga, insisted it was his responsibility to investigate "corruption."
"I don't care about Biden's campaign, but I care about corruption," he told reporters.
Biden, the former US vice president, responded by calling Trump "the most corrupt president we've had in modern history."
"I am not going to stand for it," Biden said at a campaign event in Los Angeles.
"He has indicted himself by his own statements," Biden added, one day after Trump openly called for both Beijing and Kiev to investigate the Democrat for corruption.
Trump has alleged that Biden's son Hunter earned "millions" from sitting on the board of directors of a Ukraine tycoon's gas company.
But no evidence has surfaced showing wrongdoing by either Biden.
The diplomats' text messages showed that some had doubts about Trump's pressure on Zelensky, including allegedly freezing a $400m military aid package.
The release last week of the complaint and a summary of Trump's July 25 call with Zelensky, in which he asked for a "favour" and referred to investigating Biden, ignited the Democrats' impeachment inquiry.
As Democrats raised their attacks on Trump, many Republicans remained quietly supportive of the president.
Breaking with the pack, Senator Mitt Romney, the Republican presidential nominee in 2012, blasted Trump's comments that Beijing and Kiev should investigate Biden, calling the remarks "wrong and appalling."