The Trump administration complained on Saturday of harassment by Democratic lawmakers who issued subpoenas for 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 whether to come forward.

The latest explosive turns in the impeachment investigation against Trump came as Secretary of State Mike Pompeo accused Democratic-led congressional committees leading the probe 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".

He called the impeachment drama "some silly gotcha game," saying the matter distracts from important issues like improving lives and growing economies.

Read: Pompeo subpoenaed as Democrats press on with impeachment probe

The House committees issued their subpoenas — including to the White House — as evidence mounted that Trump abused his powers by attempting to withhold US military aid to pressure Ukraine President Volodymyr Zelensky into seeking damaging information on 2020 Democratic rival Joe Biden.

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 impeachment investigation saga began after an original whistleblower — reportedly a CIA analyst — filed a formal complaint to the intelligence community inspector general about Trump's alleged pressuring of Zelensky.

"Look, nations do this," Pompeo said on Saturday in Athens.

"Nations work together and they say, 'Boy, goodness gracious, if you can help me with X, we'll help you achieve Y.' This is what partnerships do. It's win-win."

'Keep them coming!'

Trump and his supporters have aggressively attacked the still-anonymous whistleblower whom Trump has denounced as "close to a spy".

But if a second whistleblower emerges, and if that person has more direct information about the events in question, as The New York Times reported late on Friday, the allegations would become more difficult to shrug off.

However, the possibility did not seem to deter Trump, who welcomed the idea of another whistleblower, stating on Twitter: "Keep them coming!"

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.

In a tweet late on Saturday, Trump said that the first whistleblower "got my phone conversation almost completely wrong, so now word is they are going to the bench and another 'Whistleblower' is coming in from the Deep State, also with second hand info".

Congressional investigators led by House Intelligence Committee chairman Adam Schiff have promised a quick and expeditious inquiry, but said on Friday the White House was not cooperating with their demand for documents.

"The White House has refused to engage with — or even respond to — multiple requests for documents," said a letter signed by the chairmen of the House oversight, intelligence and foreign affairs committees.

"His actions have left us with no choice but to issue this subpoena."

The committees ordered the White House to turn over 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 a key Trump-Zelensky phone call on July 25 — failed to meet a Friday subpoena deadline to turn over Ukraine-related documents, CNN reported, citing a House foreign affairs committee aide.

Republicans quiet

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.

The text messages showed that some US diplomats had doubts about Trump's pressure on Zelensky, including allegedly freezing a $400 million military aid package.

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 after the US leader openly called on Thursday for Beijing and Kiev to investigate Biden. Romney called the remarks "wrong and appalling".

Senator Susan Collins on Saturday joined Romney in condemning Trump's appeal to China.

"I thought the president made a big mistake by asking China to get involved in investigating a political opponent," she told reporters in Maine, according to the Bangor Daily News. "It's completely inappropriate."