- Video report by ITV News Washington Correspondent Robert Moore
Donald Trump has been warned against retaliating as he lashed out at anyone who might have helped an intelligence whistleblower whose complaint is at the centre of the impeachment probe.
It comes as Democrat lawmakers have ordered the Secretary of State Mike Pompeo to give evidence of what he knew about President Trump's dealings with Ukraine.
They're investigating a whistleblower's complaint which alleged that the US president abused the power of his office by asking the Ukraine leader to look into his leading political rival Joe Biden in return for military aid.
But away from Capitol Hill, Mr Trump's supporters are rallying round, and are as vocal as his critics.
Sheriffs across the US, who gathered at the White House on Thursday cheered and shouted "We've got your back".
ITV News Washington Correspondent Robert Moore says: "He still enjoys extraordinary supports from such groups even if he is facing mounting political peril."
In a July 25 phone call, days after ordering a freeze to some military assistance for Ukraine, Mr Trump prodded new President Volodymyr Zelenskiy to investigate Democratic rival Joe Biden and volunteered the assistance of his personal lawyer Rudy Giuliani and attorney general William Barr.
Late on Thursday, Mr Trump denounced people who might have talked to the whistleblower as "close to a spy" and suggested they engaged in treason, an act punishable by death.
On Friday, he targeted the complainant, a CIA officer, tweeting: "Sounding more and more like the so-called Whistleblower isn’t a Whistleblower at all."
Nancy Pelosi, speaker of the House of Representatives, told MSNBC’s Morning Joe show: "I’m concerned about some of the president’s comments about the whistleblower."
She said House panels conducting the impeachment probe will make sure there is no retaliation against people who provided information in the case.
On Thursday, House Democratic chairmen called Mr Trump’s comments "witness intimidation" and suggested efforts by him to interfere with the potential witness could be unlawful.
Mr Trump’s Friday comment questioning the whistleblower’s status could foreshadow an effort to argue that legal protection laws do not apply to the person, opening a new front in the president’s battles with Congress.
The intelligence community’s inspector general found the whistleblower’s complaint "credible" despite finding indications of the person’s support for a different political candidate.
As more Democrats have lent support to investigations that could result in the removal of the president, Ms Pelosi has moved to focus the probe on the Ukraine matter, rather than the array of other open inquiries.
"I think we have to stay focused, as far as the public is concerned, on the fact that the president of the United States used taxpayer dollars to shake down the leader of another country for his own political gain," she said on Friday.
She declined to provide a timeline for the House impeachment investigation.
"They will take the time that they need, and we won’t have the calendar be the arbiter," she said. But she added: "It doesn’t have to drag on."
Republicans are straining under the uncertainty of being swept up in the most serious test yet of their alliance with the Trump White House.
"We owe people to take it seriously," said senator Marco Rubio, a one-time Trump rival who is now a member of the intelligence committee.
"Right now, I have more questions than answers,” he said. “The complaint raises serious allegations, and we need to determine whether they’re credible or not."
Fresh questions were raised late on Thursday about how the White House and the Justice Department handled the whistleblower complaint.
The administration initially blocked Congress from viewing it, and only released a redacted version to legislators this week after the impeachment inquiry had begun.
White House and Justice Department lawyers were aware of the concerns about Mr Trump’s call with Mr Zelenskiy before the complaint was filed, according to sources.
The intelligence official initially filed a complaint about Mr Trump’s dealings with Ukraine with the CIA, which then alerted the White House and the Justice Department, before filing with the intelligence community’s inspector general, a process that granted the individual more legal protection.
Ukraine’s top anti-corruption official said on Friday that his agency has not investigated Mr Biden.