A second whistleblower has spoken to the U.S. intelligence community's internal watchdog and has information that backs the original whistleblower's complaint about President Donald Trump's dealings with Ukraine, according to the lawyer for the pair.

Mark Zaid told The Associated Press in a text message Sunday that the second whistleblower, who also works in intelligence, hasn't filed a complaint with the inspector general but does have "first-hand knowledge that supported" the original whistleblower.

The original whistleblower, a CIA officer, filed a formal complaint with the inspector general on Aug. 12, ultimately triggering the impeachment inquiry currently being led by House Democrats. The complaint alleged Trump was "using the power of his office to solicit interference from a foreign country" in the 2020 election.

The disclosure of a second whistleblower threatens to undermine arguments made by Trump and his allies against the first whistleblower: That the complaint was improperly filed because it was based on second- or third-hand information.

Trump and his supporters have rejected accusations he did anything improper. But the White House has struggled to come up with a unified response. No administration officials appeared on the Sunday news shows, but several congressional Republicans came to the president's defence during television interviews.

Rep. Jim Jordan, of Ohio, and Sen. Lindsey Graham, of South Carolina, two of Trump's most vocal backers, sharply criticized the way House Democrats are handling the impeachment inquiry.

When it comes to “more whistleblowers coming forward”..............I’ve seen this movie before — with Brett <a href="https://twitter.com/hashtag/Kavanaugh?src=hash&ref_src=twsrc%5Etfw">#Kavanaugh</a>.<br><br>More and more doesn’t mean better or reliable.<a href="https://t.co/tx1aIBYrZQ">https://t.co/tx1aIBYrZQ</a> —@LindseyGrahamSC

Graham said there was nothing wrong with Trump's July phone call during which the president pressed Ukrainian President Volodymyr Zelensky to investigate former vice-president Joe Biden and his son, Hunter. The conversation has raised questions about whether Trump was using near $400 million US in critical American military aid to Ukraine as leverage to get help on the Biden issue.

Hunter Biden served on the board of Burisma, a Ukrainian gas company, 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 is currently running for the 2020 Democratic presidential nomination.

"I think this is a nightmare for the Biden campaign," Graham said.

In an opinion piece for The Washington Post published over the weekend, Biden wrote that he had a message for 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."

Trump lashes out on Twitter

As for Trump, rather than visiting his golf course in Sterling, Va., for a second day, he stayed at White House Sunday, where he tweeted and retweeted, with the Bidens serving as a main target.

"The great Scam is being revealed!" Trump wrote at one point, continuing to paint himself as the victim of a "deep state" and hostile Democrats, even after standing on the South Lawn last week and publicly calling on another foreign government, China, to investigate Biden.

As the president often does when he feels under attack, he trumpeted his strong support among Republican voters and kept lashing out at Utah Sen. Mitt Romney, one of the few Republicans who has publicly questioned Trump's conduct.

"The Democrats are lucky that they don't have any Mitt Romney types," Trump wrote, painting the former presidential candidate as a traitor to his party. Romney had said on Twitter that, "By all appearances, the President's brazen and unprecedented appeal to China and to Ukraine to investigate Joe Biden is wrong and appalling."

Trump again lashed out at Utah Sen. Mitt Romney, one of the few Republicans who has publicly questioned the president's conduct. (Patrick Semansky/Associated Press)

A Democrat on the House intelligence committee, Florida Rep. Val Demings, said she believes the original whistleblower is a "patriot" who stepped forward to report wrongdoing despite the potential career risk.

"The reporting that a second whistleblower has come forward or is about to come forward, I believe again would be someone who sees wrongdoing, hears wrongdoing and wants to do something about it," Demings said.

In response to news of an additional whistleblower, Sen. Amy Klobuchar, of Minnesota, another Democratic presidential candidate, said Trump is "acting like a global gangster."

Additional details about Trump's July 25 call with Zelensky emerged Sunday.

Shaylyn Hynes, a spokesperson for Energy Secretary Rick Perry, said Perry had encouraged Trump to speak with the Ukrainian leader — but on energy and economic issues.

Trump, who has repeatedly described his conversation with Zelensky as "perfect," told House Republicans on Friday night that it was Perry who teed up that call, according to a person familiar with Trump's comments who was granted anonymity to discuss them. The person said Trump did not suggest that Perry had anything to do with the pressure to investigate the Bidens.

Hynes said Perry's interest in Ukraine is part of U.S. efforts to boost Western energy ties to Eastern Europe.

Demings was on Fox News Sunday and Jordan appeared on ABC's This Week, while Klobuchar was on CNN's State of the Union and Graham spoke on Fox News Channel's Sunday Morning Futures.

Watch: What does it take to impeach a president?