A second whistleblower has come forward, this one with first-hand information of the events that triggered an impeachment investigation of President Donald Trump for alleged abuse of power, the informant's lawyer said Sunday.

"I can confirm this report of a second #whistleblower being represented by our legal team," Mark Zaid said on Twitter. "They also made a protected disclosure under the law and cannot be retaliated against. This WBer has first-hand knowledge."

Earlier Sunday, Zaid's co-counsel, Andrew Bakaj, said his firm and team "represent multiple whistleblowers" in the case accusing Trump of using the powers of his office to pressure Ukrainian president Volodymyr Zelensky in a phone call to investigate political rival Joe Biden and his son Hunter.

It was unclear whether Bakaj was using "multiple" to refer to more than two whistleblowers. Typically, several officials would listen in on a call between the president and a foreign leader, while others would have access to a written transcript or summary.

The existence of a whistleblower claiming first-hand knowledge would make it harder for the president and his supporters to dismiss the original complaint as hearsay, as they have repeatedly done.

- Trump pushes back -

Trump pushed back at the allegations in two tweets early Sunday, though he made no mention of the second whistleblower.

He repeated his assertion that Hunter Biden had been "handed $100,000 a month (Plus,Plus) from a Ukrainian based company, even though he had no experience in energy...and separately got 1.5 Billion Dollars from China despite no experience and for no apparent reason."

He added that as president, "I have an OBLIGATION to look into possible, or probable, CORRUPTION!"

Other reports have said Hunter Biden was paid up to $50,000 a month as a member of the board of a Ukrainian gas company, Burisma.

No evidence has been found that either Biden did anything illegal.

A bit unusually for a Sunday, Trump was staying in the White House rather than traveling or playing golf.

"On one of the most critical news weeks of the last three years," CNN said in a tweet quoting anchor Jake Tapper, "the White House did not offer a guest, the President's personal lawyers and Congressional GOP leaders either declined to be on the show or did not respond."

But one Republican senator, Ron Johnson, who serves on the Foreign Relations Committee, appeared on NBC's "Meet the Press" and rejected the suggestion that Trump had withheld military aid to pressure Ukraine to investigate the Bidens.

"When I asked the president about that," he said, "he completely adamantly, vehemently, angrily denied it."

- 'Harassment' -

The latest turns in the explosive impeachment inquiry came a day after 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.

The House committees issued subpoenas on Friday -- including to the White House -- as evidence mounted that Trump attempted to withhold US military aid to pressure Zelensky into seeking damaging information on Biden, who has led in most polls of 2020 Democratic presidential aspirants.

The impeachment investigation saga began after the original whistleblower -- an intelligence official -- filed a formal complaint to the intelligence community inspector general about Trump's alleged pressuring of Zelensky.

A rough transcript of the phone call later released by the White House, as well as a series of text messages between US diplomats, appeared to corroborate the original complaint.

Zaid recently told the Washingtonian magazine that he hoped the identity of the original whistleblower -- whom Trump has assailed as treasonous -- would never become public.

His co-counsel, Bakaj, previously worked in the inspector general's offices at both the CIA and the Defense Department on whistleblower-related issues. (AFP)

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