US Democrats leading an impeachment inquiry into President Donald Trump’s dealings with Ukraine may have fresh information to work with after a new whistleblower stepped forward.

With US congress out for another week and many Republicans reluctant to speak out, a lawyer for a second individual said they could corroborate the original whistleblower’s complaint.

Attorney Mark Zaid, who represents both whistleblowers, told the Associated Press that the new whistleblower works in the intelligence field and has spoken to the intelligence community’s internal watchdog.

Joe Biden
Joe Biden, one of the frontrunners in the race to challenge Mr Trump for the presidency next year (AP)

The original whistleblower, a CIA officer, filed the formal complaint with the inspector general in August that triggered the impeachment inquiry.

The document alleged that Mr Trump had used a July telephone call with Ukrainian president Volodymyr Zelenskiy to investigate a political rival, Joe Biden, and his son Hunter, prompting a White House cover-up.

The push came even though there was no evidence of wrongdoing by the former vice president or his son, Hunter, who served on the board of a Ukrainian gas company.

Mr Trump and his supporters deny that he did anything improper, but the White House has struggled to come up with a unified response.

A second whistleblower with direct knowledge could undermine efforts by Mr Trump and his allies to discredit the original complaint.

Volodymyr Zelenskiy
Ukrainian president Volodymyr Zelenskiy (AP)

They have called the action politically motivated, saying it was filed improperly and dismissing it as unreliable because it was based on second-hand or third-hand information.

A rough transcript of Mr Trump’s call with Mr Zelenskiy, released by the White House, has already corroborated the complaint’s central claim that the US president sought to put pressure on Ukraine over the investigation.

Text messages from US state department officials revealed other details, including that Ukraine was promised a visit with Mr Trump if the government would agree to investigate the 2016 election and a Ukrainian gas company tied to Mr Biden’s son – the outline of a potential quid pro quo.

Democratic representative Jim Himes, a member of the House intelligence committee, said word of a second whistleblower indicates a larger shift inside the government.

Mr Himes said: “The president’s real problem is that his behaviour has finally gotten to a place where people are saying: ‘Enough.'”

Democrats have zeroed in on the state department in the opening phase of their impeachment investigation.

The intelligence, oversight and foreign affairs committees have already interviewed Kurt Volker, a former special envoy to Ukraine who provided the text messages, and least two other witnesses are set for depositions this week: Gordon Sondland, the US ambassador to the European Union, and Marie Yovanovitch, who was abruptly ousted as the US ambassador to Ukraine in May.

Senator Lindsey Graham, one of Mr Trump’s most vocal backers, provided perhaps the strongest defence of the president. He said there was nothing wrong with Mr Trump’s July conversation with Mr Zelenskiy and said the accusations look like a “political set-up”.

As for Mr Trump, rather than visiting his nearby golf course in Sterling, Virginia, for a second day, he stayed at the White House on Sunday, where he fired out several tweets and retweets with the Bidens as a main target.

“The great Scam is being revealed!” he wrote at one point, continuing to paint himself as the victim of a “deep state” and hostile Democrats.

Aside from Mr Trump’s attempt to pressure Mr Zelenskiy, the July call has raised questions about whether Mr Trump held back close to 400 million dollars in critical American military aid to Ukraine as leverage for an investigation of Burisma, a Ukrainian gas company.

Hunter Biden served on the board of Burisma at the same time his father was leading the Obama administration’s diplomatic dealings with Ukraine.

Though the timing raised concerns among anti-corruption advocates, there has been no evidence of wrongdoing by either Biden.

Joe Biden, a leading candidate for the 2020 Democratic presidential nomination, wrote in The Washington Post that he had a message for Mr Trump and “those who facilitate his abuses of power. … Please know that I’m not going anywhere. You won’t destroy me, and you won’t destroy my family”.