Democrats are urging people who might have more information about US President Donald Trump's effort to persuade Ukraine to investigate political rival Joe Biden to come forward as Mr Trump struggles to contain the fallout from the scandal.
A whistleblower report, released on Thursday, said Mr Trump not only abused his office in attempting to solicit Ukraine's interference in the 2020 US election for his political benefit, but that the White House tried to "lock down" evidence about that conduct.
At issue is a 25 July phone call that Mr Trump had with Ukrainian President Volodymyr Zelenskiy to congratulate him on his election victory.
In the call, according to a summary released by the Trump administration on Wednesday, Mr Trump asked Zelenskiy to investigate Biden and his son Hunter, who worked for a Ukrainian gas company for several years.
Mr Trump has repeatedly suggested wrongdoing by Mr Biden and his son, but has offered no evidence to back up the assertion.
President Donald Trump has been accused of using the power of his office to solicit interference from a foreign country. Source: AAP
Mr Biden, a former vice president, is a leading contender in the Democratic race to take on Mr Trump in the 2020 presidential election.
A growing number of Democrats say the call was an abuse of Mr Trump's position and want to see him impeached. But the chances of the Republican president being removed from office look slim since his party controls the Senate where an impeachment trial would be held.
Democrat Adam Schiff, who chairs the House of Representatives Intelligence Committee and is one of Mr Trump's biggest opponents, called for more people to come forward who have knowledge of the Ukraine case.
"I'm hoping that these witnesses will choose to cooperate, will volunteer. But I have to say I am deeply worried now that the president, on the eve of our hearing or during our hearing, was threatening these witnesses," Mr Schiff told CNN on Thursday.
"It's another witch hunt. Here we go again," Mr Trump told reporters on Thursday, assailing House Speaker Nancy Pelosi's Democrats for launching an impeachment inquiry into him this week.
Adam Schiff has called for anyone with more information about Donald Trump's phone call with the Ukrainian leader to come forward. Source: AAP
Mr Trump and his Republican allies say he was simply leaning on Mr Zelenskiy to carry out his own campaign promise to root out corruption in Ukraine.
Still, Mr Trump's political allies are worried, given that the impeachment inquiry will likely lead to weeks or months of negative media coverage of Mr Trump and distract from his re-election bid.
The inquiry is casting a new pall over his presidency just months after he emerged from the dark cloud cast by Special Counsel Robert Mueller's investigation into whether he colluded with Russia in the 2016 election.
Three US House committees said they would issue subpoenas to the White House and State Department as soon as Friday if the Trump administration misses a deadline to send a wide range of documents related to its dealings with Ukraine.
